Chris Huhne dramatically announced he was stepping down from Parliament after pleading guilty to perverting the course of justice.
The former energy secretary was due to stand trial alongside ex-wife Vicky Pryce on charges that he made her take speeding points on his behalf.
But he dramatically changed his plea as the case was about to open at London's Southwark Crown Court.
Making a statement outside court afterwards, the Liberal Democrat said: "Having taken responsibility for something that happened 10 years ago, the only proper course of action for me is to resign my Eastleigh seat in Parliament, which I will do shortly."
Prime Minister David Cameron's official spokesman declined to comment on the former minister's resignation, saying only: "It is a matter for Mr Huhne." Lib Dem leader Nick Clegg is understood to be "shocked and saddened" at the development, and is expected to make a statement soon.
Huhne's departure from the Commons is the latest humiliation in a tumultuous fall from grace that began when allegations over the speeding penalty surfaced early in 2011. He was only narrowly beaten by Mr Clegg for the party leadership in 2007 and was still regarded by many as the Deputy Prime Minister's natural successor.
The 58-year-old strongly denied the claims when he was charged by prosecutors last February and repeated the denials in court as recently as last week. But when asked again, he clearly stated: "Guilty."
PR adviser Carina Trimingham was in the public gallery to watch as Huhne made the admission. The emergence of their relationship in September 2010 ended his marriage to 60-year-old Pryce, and the speeding allegations leaked out soon afterwards.
Perverting the course of justice carries a maximum sentence of life - but the average is around 10 months. Under Commons rules, MPs are only forced to step down if they receive custodial terms of more than 12 months, but realistically Huhne's departure was regarded as inevitable.
Pryce's case was adjourned until 10am on Tuesday at Southwark, when a trial is expected to start. She was granted unconditional bail.