The DUP's David Simpson is standing down as MP for Upper Bann after a reported affair with a party colleague.
He first won the seat in 2005 and has been a member of the party for decades.
In a statement, he said: "In recent times it has been well publicised that I made a mistake that brought hurt to my wife and family as well as bringing heartache to another family."
Mr Simpson said he was sorry for his "mistakes" and that his family had given him a "precious second chance".
"These were my mistakes and for them I am truly sorry," he said.
"I have learned from my failings and my wife and children have given me a precious second chance and together we intend to look to the future together."
Before becoming an MP, Mr Simpson was a member of the Stormont assembly and had also been a councillor for the party.
He said the time was right to "pass the baton on".
DUP leader Arlene Foster thanked Mr Simpson for his years of consistent public service to voters and the party.
"His service has been characterised by dedication to the people who elected him and his business skills and experience were used to the benefit of Upper Bann," she said.
"I wish him well as he takes a new direction in life and I know that he will continue to use his many talents to the benefit of the party and the wider Northern Ireland cause."
It was a dramatic result in 2005, when he first snatched the Upper Bann seat from the then-Ulster Unionist leader David Trimble.
It proved to be a landmark victory that showed how the DUP had eclipsed the UUP.
Before becoming a politician, Mr Simpson had worked in his own meat business but came to prominence as the mayor of Craigavon and later an MLA.
Away from the political world, he was a keen preacher and gospel singer.
'Adam and Steve'
In Parliament, he was not regarded as one of the party's high-flyers but was a regular contributor in the Commons and a member of the Northern Ireland Affairs Committee.
He was opposed to same-sex marriage, but in 2013 during a debate in Parliament on legalising it, he said: "In the Garden of Eden it was Adam and Steve" before correcting himself to "Adam and Eve".
In 2017, he claimed in election literature that he had visited soldiers in Afghanistan in 2010, but it later emerged this had not happened because he could not fit into a flak jacket.
It is not clear who the party will select to replace him to stand as a candidate for Upper Bann in the 12 December general election.
Nominations close on 14 November.