Conservative Keith Simpson says he will not stand at the next general election after 22 years in the House of Commons.
The MP for Broadland in Norfolk said the decision had "nothing to do with Brexit", but because he had reached the "ripe old age" of 70.
He told his local party in August to give time to nominate a successor in the event of snap election.
Mr Simpson voted against the government in July in an attempt to block a no-deal Brexit.
He said he had not yet decided whether to support a bill in the Commons on Tuesday to stop no deal.
Goodnight from him
Announcing his decision to stand down, he said: "I will not be standing at the next general election.
"This has got nothing to do with Brexit. I'm the ripe old age of 70 and I decided some time ago that I would not stand again.
"You should quit the stage while people are saying 'more, more, more'. The late Ronnie Barker used to say that.
"It's not how old you are now but, if you got re-elected, how old you would be in five years' time."
Mr Simpson was elected MP for Mid Norfolk in 1997, and then Broadland from 2010 after constituency boundaries were redrawn. He held the seat at the 2017 general election with a majority of 15,816.
A military historian, he served as shadow defence minister between 2002 and 2005, and shadow minister for foreign affairs from 2005 to 2010.
He has also sat on the intelligence and security committee since 2015.