The Lieutenant of the Admiralty is a now honorary office generally held by a senior retired Royal Navy admiral. He is the official deputy to the Lord High Admiral, an office vested in Prince Philip, Duke of Edinburgh since 2011. He is appointed by the Sovereign on the nomination of the First Sea Lord. The office was created in 1545 by Henry VIII. Amongst other responsibilities, the Lieutenant of the Admiralty presided over the Council of the Marine, later known as the Navy Board.
From 1801 the office was joined with that of Vice-Admiral of the United Kingdom, and was the second most powerful position in the Royal Navy.
Below the office of Vice-Admiral is the Rear-Admiral of the United Kingdom, another now honorary office.
References
Lieutenant of the Admiralty Wikipedia(Text) CC BY-SA