The Westminster St. George's by-election, 1931 was a parliamentary by-election held on 19 March 1931 for the British House of Commons constituency of Westminster St. George's.
The seat had become vacant on 14 February when the constituency's Conservative Member of Parliament (MP), Sir Laming Worthington-Evans, died aged 62. He had sat for the constituency since the 1929 general election, having previously been MP for Colchester since 1910; he had served in the cabinets of David Lloyd George and Stanley Baldwin during the 1920s.
The by-election took place during a campaign, led by the press magnates Lord Beaverbrook and Lord Rothermere, to remove Stanley Baldwin as Leader of the Opposition. The vehicles for their campaign were the United Empire Party and the Empire Free Trade Crusade.
The campaign had had some success. The Conservative Central Office had withdrawn support for its own candidate at the 1929 Twickenham by-election, who supported the Empire Free Trade policy. The UEP had won the 1930 Paddington South by-election from the Conservatives. The split in the right-wing vote between Conservative and UEP candidates at the Islington East by-election in February 1931 had allowed Labour to hold a seat they had been expected to lose.
The industrialist Sir Ernest Willoughby Petter announced his candidacy on 28 February as an Independent Conservative opposed to Baldwin's leadership of the Conservative Party. Petter had founded the Petters Limited engineering company from which Westland Aircraft was separated in 1915. Though he claimed to be free of party and running at the request of the electors, he was eagerly backed by the Beaverbrook and Rothermere papers, the Daily Express and Daily Mail.
The Conservatives originally selected John Moore-Brabazon. He withdrew on 28 February, saying he could not defend Baldwin. Baldwin, under pressure to resign as Leader of the Conservative Party, toyed with the idea of resigning his safe Worcestershire seat of Bewdley and contesting the by-election himself.
The eventual Conservative candidate was Alfred Duff Cooper, who had been MP for Oldham from 1924 until his defeat in 1929. He had been Financial Secretary to the War Office from 1928 to 1929.
In 1929 there had been a Labour candidate for the constituency, but Labour did not contest the by-election.
One notable speech during the campaign was by Stanley Baldwin. At the Queen's Hall on 17 March he attacked the press proprietors, uttering the often-quoted words: "What the proprietorship of those papers is aiming at is power, and power without responsibility – the prerogative of the harlot through the ages".
Cooper won the by-election with 59.9% of the votes.
The Conservative victory at the by-election was an important factor in Baldwin's retention of the Conservative Party leadership. Following the collapse later that year of the Labour Government, the Conservatives would unite with the Prime Minister Ramsay MacDonald to form the National Government, which enjoyed a landslide victory at the polls that autumn.
Cooper was unopposed at the general election later that year, and remained MP for the constituency until 1945.