Johnson has accepted the post of crown steward and bailiff of the Chiltern Hundreds, the government said, part of the convoluted process under which MPs resign.
It brings to an end his eight years as MP for Uxbridge and South Ruislip in west London. Since being ousted in July last year Johnson has spent large amounts of time abroad.
Johnson’s departure as an MP is the second time he has left parliament, serving first from 2001 to 2008 as the MP for Henley until he stepped down to take over as London mayor.
Rules dating back to 1624 state that MPs can only leave their seats as a result of death, expulsion or disqualification and cannot voluntarily step down. They have to instead accept a paid crown office, which automatically disqualifies them.
“The chancellor of the exchequer has this day appointed Alexander Boris de Pfeffel Johnson to be steward and bailiff of the Three Hundreds of Chiltern,” a statement by the Treasury said.