LONDON — The post is ancient, and the duties are light (nonexistent, actually). But in order to step down from his seat in the House of Commons, David Cameron, the former prime minister of Britain, had to take on another position on Monday: Crown Steward and Bailiff of the Manor of Northstead.

Under arcane parliamentary rules, normally referred to as “taking the Chiltern Hundreds,” the purely symbolic appointment provides a legal escape hatch from the House of Commons by disqualifying lawmakers from holding their seats. Embracing the role has helped many lawmakers bring down the curtain on their careers without waiting for the next election. In this case, it is formally closing out a political rise and fall defined by Mr. Cameron’s decision to stage a referendum on European Union membership.

Mr. Cameron resigned as prime minister in June after failing to persuade Britons to vote to remain inside the bloc. The announcement on Monday means that he will also give up his parliamentary seat in Witney, Oxfordshire. The seat will be filled by a special election.

“In my view, with modern politics, with the circumstances of my resignation, it isn’t really possible to be a proper backbench M.P. as a former prime minister,” Mr. Cameron told ITV News, using the abbreviation for member of Parliament. “I think everything you do will become a big distraction and a big diversion from what the government needs to do for our country.”