Ben Wallace told the Sunday Times he would not stand at the next general election, but ruled out leaving “prematurely” and triggering a by-election.
Mr Wallace has served as defence secretary under three different PMs and played a high-profile role in the UK’s response to the war in Ukraine.
Sources told the BBC they expect the next reshuffle in September.
His Wyre and Preston North constituency is set to disappear under upcoming boundary changes and he told the newspaper he would not seek a new one.
The announcement comes after days of speculation that he might leave government after four years in the role.
He has previously served as a minister in the Northern Ireland department and in the Home Office.
The BBC understands Mr Wallace informed Prime Minister Rishi Sunak on 16 June of his decision to stand down from the Cabinet.
Mr Wallace told the Sunday Times: “I went into politics in the Scottish parliament in 1999. That’s 24 years. I’ve spent well over seven years with three phones by my bed.”
Another high-profile Tory to head for rich pickings on the lecture circuit.
