The Statute of Westminster gave effect to certain political resolutions passed by the Imperial Conferences of 1926 and 1930; in particular, the Balfour Declaration of 1926. The main effect was the removal of the ability of the British parliament to legislate for the Dominions, part of which also required the repeal of the Colonial Laws Validity Act 1865 in its application to the Dominions. King George V expressed his desire that the laws of succession be exempt from the statute's provisions, but, it was determined that would be contrary to the principles of equality set out in the Balfour Declaration. Both Canada and the Irish Free State pushed for the ability to amend the succession laws themselves and section 2(2) (allowing a Dominion to amend or repeal laws of paramount force, such as the succession laws, insofar as they are part of the law of that Dominion) was included in the Statute of Westminster at Canada's insistence. After the Statute was passed, the British parliament could no longer make laws for the Dominions, other than with the request and consent of the government of that Dominion. Before then, the Dominions had legally been self-governing colonies of the United Kingdom. However, the Statute had the effect of making them sovereign nations once they adopted it.
The Statute provides in section 4:
No Act of Parliament of the United Kingdom passed after the commencement of this Act shall extend, or be deemed to extend, to a Dominion as part of the law of that Dominion, unless it is expressly declared in that Act that that Dominion has requested, and consented to, the enactment thereof.
It also provides in section 2(1):
No law and no provision of any law made after the commencement of this Act by the Parliament of a Dominion shall be void or inoperative on the ground that it is repugnant to the Law of England, or to the provisions of any existing or future Act of Parliament of the United Kingdom, or to any order, rule or regulation made under any such Act, and the powers of the Parliament of a Dominion shall include the power to repeal or amend any such Act, order, rule or regulation in so far as the same is part of the law of the Dominion.
The whole Statute applied to Canada, the Irish Free State, and the Union of South Africa without the need for any acts of ratification; the governments of those countries gave their consent to the application of the law to their respective jurisdiction. Section 10 of the Statute provided that sections 2 to 6 would apply in the other three Dominions—Australia, New Zealand, and Newfoundland—only after the parliament of that Dominion had legislated to adopt them.
Since 1931, over a dozen new Commonwealth realms have been created, all of which now hold the same powers as the United Kingdom, Canada, Australia, and New Zealand over matters of change to the monarchy, though the Statute of Westminster is not part of their laws. Ireland and South Africa are now republics and Newfoundland is part of Canada.
Australia adopted sections 2 to 6 of the Statute of Westminster with the Statute of Westminster Adoption Act 1942, in order to clarify the validity of certain Australian legislation relating to the Second World War; the adoption was backdated to 3 September 1939, the beginning of the war.
Adopting section 2 of the Statute clarified that the Commonwealth parliament was able to legislate inconsistently with British legislation, adopting section 3 clarified that it could legislate with extraterritorial effect. Adopting section 4 clarified that Britain could legislate with effect on Australia as a whole only with Australia's request and consent.
Nonetheless, under section 9 of the Statute, on matters not within Commonwealth power Britain could still legislate with effect in all or any of the Australian states, without the agreement of the Commonwealth although only to the extent of "the constitutional practice existing before the commencement" of the Statute. However, this capacity was never used.
All British power to legislate with effect in Australia ended with the Australia Act 1986, the British version of which says that it was passed with the request and consent of the Australian parliament, which had obtained the concurrence of the Australian states.
This statute limited the legislative authority of the British parliament over Canada, effectively giving the country legal autonomy as a self-governing Dominion, though the British parliament retained the power to amend Canada's constitution at the request of the Parliament of Canada. That authority remained in effect until the Constitution Act, 1982, which transferred it to Canada, the final step to achieving full sovereignty.
The British North America Acts—the written elements (in 1931) of the Canadian constitution—were excluded from the application of the statute because of disagreements between the Canadian provinces and the federal government over how the British North America Acts could be otherwise amended. These disagreements were resolved only in time for the passage of the Canada Act 1982, thus completing the patriation of the Canadian constitution to Canada. At that time, the Canadian parliament also repealed sections 4 and 7(1) of the Statute of Westminster. The Statute of Westminster remains a part of the constitution of Canada by virtue of section 52(2)(b) of the Constitution Act, 1982.
As a consequence of the statute's adoption, the Parliament of Canada gained the ability to abolish appeals to the Judicial Committee of the Privy Council. Criminal appeals were abolished in 1933, while civil appeals continued until 1949. The passage of the Statute of Westminster meant that changes in British legislation governing the succession to the throne no longer automatically applied to Canada.
The Irish Free State never formally adopted the Statute of Westminster, its Executive Council taking the view that the Anglo-Irish Treaty of 1921 had already ended Westminster's right to legislate for the Free State. The Free State's constitution gave the Oireachtas "sole and exclusive power of making laws". Hence, even before 1931, the Free State did not arrest British Army and Royal Air Force deserters on its territory, even though the UK believed post-1922 British laws gave the Free State's Garda Síochána the power to do so. The UK's Irish Free State Constitution Act 1922 said, however, "[n]othing in the [Free State] Constitution shall be construed as prejudicing the power of [the British] Parliament to make laws affecting the Irish Free State in any case where, in accordance with constitutional practice, Parliament would make laws affecting other self-governing Dominions".
Motions of approval of the Report of the Commonwealth Conference had been passed by the Dáil and Seanad in May 1931 and the final form of the Statute of Westminster included the Irish Free State among the Dominions the British parliament could not legislate for without the Dominion's request and consent. Originally, the UK government had wanted to exclude from the Statute of Westminster the legislation underpinning the 1921 treaty, from which the Free State's constitution had emerged. President W. T. Cosgrave objected, although he promised the Executive Council would not amend the legislation unilaterally. The other Dominions backed Cosgrave and, when an amendment to similar effect was proposed at Westminster by John Gretton, parliament duly voted it down. When the Statute became law in the UK, Patrick McGilligan, the Free State Minister for External Affairs, stated: "It is a solemn declaration by the British people through their representatives in Parliament that the powers inherent in the Treaty position are what we have proclaimed them to be for the last ten years." He went on to present the Statute as largely the fruit of the Free State's efforts to secure for the other Dominions the same benefits it already enjoyed under the treaty.
After Éamon de Valera led Fianna Fáil to victory in the Free State election of 1932, he began removing the monarchical elements of the constitution, beginning with the Oath of Allegiance. De Valera initially considered invoking the Statute of Westminster in making these changes, but John J. Hearne advised him not to. Abolishing the Oath of Allegiance in effect abrogated the 1921 treaty. Generally, the British thought that this was morally objectionable but legally permitted by the Statute of Westminster. Robert Lyon Moore, a southern unionist from County Donegal, challenged the legality of the abolition in the Free State courts and then appealed to the Judicial Committee of the Privy Council (JCPC) in London. However, the Free State had also abolished the right of appeal to the JCPC. In 1935, the JCPC ruled that both abolitions were valid under the Statute of Westminster. The Free State, which in 1937 was renamed Ireland, left the Commonwealth in 1949 upon the coming into force of its Republic of Ireland Act.
The Parliament of New Zealand adopted the Statute of Westminster by passing its Statute of Westminster Adoption Act 1947 in November 1947. The New Zealand Constitution Amendment Act, passed the same year, empowered the New Zealand parliament to change the constitution, but did not remove the ability of the British parliament to legislate regarding the New Zealand constitution. The remaining role of the British parliament was removed by the New Zealand Constitution Act 1986 and the Statute of Westminster was repealed in its entirety.
The Dominion of Newfoundland never adopted the Statute of Westminster, especially because of financial troubles and corruption there. By request of the Dominion's government, the United Kingdom established the Commission of Government in 1934, resuming direct rule of Newfoundland. That arrangement remained until Newfoundland became a province of Canada in 1949.
Although the Union of South Africa was not among the Dominions that needed to adopt the Statute of Westminster for it to take effect, two laws—the Status of the Union Act, 1934, and the Royal Executive Functions and Seals Act of 1934—were passed to confirm South Africa's status as a sovereign state.
The preamble to the Statute of Westminster sets out conventions which affect attempts to change the rules of succession to the Crown. The second paragraph of the preamble to the Statute reads:
And whereas it is meet and proper to set out by way of preamble to this Act that, inasmuch as the Crown is the symbol of the free association of the members of the British Commonwealth of Nations, and as they are united by a common allegiance to the Crown, it would be in accord with the established constitutional position of all the members of the Commonwealth in relation to one another that any alteration in the law touching the Succession to the Throne or the Royal Style and Titles shall hereafter require the assent as well of the Parliaments of all the Dominions as of the Parliament of the United Kingdom:
This means, for example, that any change in any realm to the Act of Settlement's provisions barring Roman Catholics from the throne would require the unanimous assent of the parliaments of all the other Commonwealth realms if the shared aspect of the Crown is to be retained. The preamble does not itself contain enforceable provisions, it merely expresses a constitutional convention, albeit one fundamental to the basis of the relationship between the Commonwealth realms. (As sovereign nations, each is free to withdraw from the arrangement, using their respective process for constitutional amendment.) Additionally, per section 4, if a realm wished for a British act amending the Act of Settlement in the UK to become part of that realm's laws, thereby amending the Act of Settlement in that realm, it would have to request and consent to the British act and the British act would have to state that such request and consent had been given. Section 4 of the Statute of Westminster has been repealed in a number of realms, however, and replaced by other constitutional clauses absolutely disallowing the British parliament from legislating for those realms.
This has raised some logistical concerns, as it would mean multiple parliaments would all have to assent to any future changes in any realm to its line of succession, as with the Perth Agreement's proposals to abolish male-preference primogeniture.
During the abdication crisis in 1936, British Prime Minister Stanley Baldwin consulted the Commonwealth prime ministers at the request of King Edward VIII. The King wanted to marry Wallis Simpson, whom Baldwin and other British politicians considered unacceptable as queen, as she was an American divorcée. Baldwin was able to get the then five Dominion prime ministers to agree with this and thus register their official disapproval at the King's planned marriage. The King later requested the Commonwealth prime ministers be consulted on a compromise plan, in which he would wed Simpson under a morganatic marriage pursuant to which she would not become queen. Under Baldwin's pressure, this plan was also rejected by the Dominions. All of these negotiations occurred at a diplomatic level and never went to the Commonwealth parliaments. However, the enabling legislation that allowed for the actual abdication (His Majesty's Declaration of Abdication Act 1936) did require the assent of each Dominion parliament to be passed and the request and consent of the Dominion governments so as to allow it to be part of the law of each Dominion. For expediency and to avoid embarrassment, the British government had suggested the Dominion governments regard whomever is monarch of the UK to automatically be their monarch. However, the Dominions rejected this; Prime Minister of Canada William Lyon Mackenzie King pointed out that the Statute of Westminster required Canada's request and consent to any legislation passed by the British parliament before it could become part of Canada's laws and affect the line of succession in Canada. The text of the British act states that Canada requested and consented (the only Dominion to formally do both) to the act applying in Canada under the Statute of Westminster, while Australia, New Zealand, and the Union of South Africa simply assented.
In February 1937, the South African parliament formally gave its assent by passing His Majesty King Edward the Eighth's Abdication Act, 1937, which declared that Edward had abdicated on 10 December 1936; that he and his descendants, if any, would have no right of succession to the throne; and that the Royal Marriages Act 1772 would not apply to him or his descendants, if any. The move was largely done for symbolic purposes, in an attempt by Prime Minister J. B. M. Hertzog to assert South Africa's independence from Britain. In Canada, the federal parliament passed the Succession to the Throne Act 1937, to assent to His Majesty's Declaration of Abdication Act and ratify the government's request and consent to it. In the Irish Free State, Prime Minister Éamon de Valera used the departure of Edward as an opportunity to remove all explicit mention of the monarch from the constitution of the Irish Free State, through the Constitution (Amendment No. 27) Act 1936, passed on 11 December 1936. The following day, the External Relations Act provided for the king to carry out certain diplomatic functions, if authorised by law. A new Constitution of Ireland, with a president, was approved by Irish voters in 1937, with the Irish Free State becoming simply "Ireland", or, in the Irish language, "Éire". However, the head of state of Ireland remained unclear until 1949, when Ireland unambiguously became a republic outside the Commonwealth of Nations by enacting the Republic of Ireland Act 1948.
In some countries where the Statute of Westminster forms a part of the constitution, the anniversary of the date of the passage of the original British statute is commemorated as Statute of Westminster Day. In Canada, it is mandated that, on 11 December, the Royal Union Flag (as the Union Jack is called by law in Canada) is to be flown at properties owned by the federal Crown, where the requisite second flag pole is available.