Former Lancaster and current Garstang MP Ben Wallace says he will step down at the next election.
Mr Wallace, who has also served as Secretary of State for Defence since 2019, said in an interview with The Times: "I'm not standing next time."
He added that he will not force a by-election by resigning "prematurely" - as fellow allies of Boris Johnson have done.
Mr Wallace had been facing the prospect of effectively losing his constituency of Wyre and Preston North at the next general election under boundary changes - meaning he would have needed to stand in another seat to remain as an MP.
Related story: Major changes for local MPs as election boundary shake-up confirms new constituencies
Ben Wallace was elected as Member of Parliament for the Lancaster and Wyre constituency at the 2005 general election. The constituency was abolished for the 2010 general election, and Wallace was elected for the new seat of Wyre and Preston North, winning re-election at the 2015, 2017, and 2019 general elections with healthy majorities.
Mr Wallace also confirmed he would leave the cabinet at the next reshuffle, which is expected to be held this autumn.
"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," he told The Times.
Speculation about the defence secretary's fate has been mounting for weeks, with officials inside the Ministry of Defence wondering who might replace him.
It also comes following a failed UK bid to make Mr Wallace the next head of NATO.
Mr Wallace told The Times that a desire to spend more time with his family, including his three children, was one of his reasons for leaving politics.
Asked what he would do next, he replied: "I'm quite happy to go and work at a bar," or "just do something completely different."
He then served as a captain in the Scots Guards and worked in the aerospace industry before entering politics in 1999.
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