Ron Davies is giving up his Welsh Assembly seat following controversy over newspaper reports of a woodland sex liaison.
The Labour former Welsh Secretary said he will not seek re-nomination in Caerphilly at the May election because he would be "a distraction".
Mr Davies, 56, denies The Sun's claims he engaged in a sex act with a builder at a picnic area near Bath on Monday.
He initially denied being anywhere near the area. But support for the "architect of devolution" dwindled after he revised his story to admit he was there - to stretch his legs and visit the lavatory.
After considering his future over the weekend following meetings with Labour bosses, he announced his decision on Sunday, saying it had been "very traumatic".
He said: "If I was the candidate, we would have this continuing tabloid focus on me. That's a distraction ... it would probably be divisive.
I'm not surprised at this conclusion - I think he's made the right decision
Jeff Cuthbert, Caerphilly Labour chairman
"In the best interest of the Labour Party, we have to have a united party where the party can campaign on the issues.
"I want the party to get behind one candidate and fight the campaign and win. The last week has been a pretty horrific time for me."
It will be the second time Mr Davies has left front-line politics, following 1998's "moment of madness" on Clapham Common which wrecked a certain future as the assembly's inaugural first secretary.
It leaves the Caerphilly party looking for a new candidate before the 1 April deadline for nominations.
Caerphilly Labour chairman Jeff Cuthbert, who met Mr Davies on Friday, said: "I'm not surprised at this conclusion.
"Under the circumstances, I think he's made the right decision.
"But we are very, very sad that someone with such a distinguished political career has come to this sort of end."
Mr Cuthbert said he would take advice on filling Mr Davies' shoes from the national Wales Labour Party, which wants selection of a new candidate to be "swift".
As Welsh Secretary in 1997, Mr Davies spearheaded the campaign for devolution, which was won with a slim referendum majority.
He looked certain to become Wales' inaugural first secretary when events surrounding his private life forced him to resign from Tony Blair's cabinet.
He left the front line after earlier revelations
He later admitted he was bisexual and had undergone psychiatric treatment for a compulsive disorder which made him seek out high-risk situations.
The revelation cost him his chairmanship of the assembly's economic development committee and he has since been a backbencher.
Current Welsh Secretary Peter Hain said: "This is a sad decision. "I think that Ron has done the right thing.
"It is a tragic end to an impressive political career. Ron made a major contribution to public life in Wales over the years."
Wales Labour Party general secretary Jessica Morden said "has done the right thing for himself" and the party.
"Caerphilly people have supported the Labour Party consistently, and we want to earn their support again and retain this seat," she said.