Chris Huhne has resigned as an MP after he admitted perverting the course of justice over claims his ex-wife took speeding points for him a decade ago.
The ex-Lib Dem cabinet minister changed his plea on the first day of his trial at Southwark Crown Court after failing in a bid to have the case dismissed.
His former wife, Vicky Pryce, has pleaded not guilty to the same offence.
The judge ruled that text messages from Huhne's son urging him to admit he was the driver could be used in evidence.
Mr Justice Sweeney revealed Huhne's legal team had previously made two applications to get the case dismissed or suspended (stayed), but these were not granted, allowing the trial to proceed.
Speaking outside court, Huhne 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."
Lib Dem leader Nick Clegg said he was "shocked and saddened" by Huhne's admission but said his former challenger for the party leadership had "taken the right decision in resigning as an MP".
BBC chief political correspondent Norman Smith said that by standing down, Huhne - a Lib Dem "big hitter" - was accepting that his career was over.
Huhne and Ms Pryce were charged last year over an incident in March 2003 when Huhne's car was allegedly caught by a speed camera on the motorway between Stansted Airport in Essex and London.
It is alleged that between 12 March and 21 May 2003, Ms Pryce, 60, from Clapham in south London, who was still married to Huhne, falsely informed police that she was the driver of the car so he could avoid prosecution.
The prosecution revealed Huhne had a history of speeding and was in danger of losing his licence, having already accrued nine penalty points.
Huhne had vowed to fight the charges and last week pleaded not guilty.
His barrister, John Kelsey-Fry QC, had previously attempted to get the Huhne case thrown out of court, describing the prosecution's evidence as "insufficient", "circumstantial" and "at best gossamer thin".
He also claimed the media had constantly assumed Huhne was guilty, which meant the ex-minister could not possibly receive a fair trial.
But text messages between Huhne and his son Peter, sent in May 2011 and declared admissible in court by Mr Justice Sweeney, revealed Peter put pressure on Huhne to "accept responsibility" for the offence.
Peter said: "We all know that you were driving and you put pressure on Mum. Accept it or face the consequences. You've told me that was the case. Or will this be another lie?"
Huhne replied: "I have no intention of sending Mum to Holloway Prison for three months. Dad"
His son asked: "Are you going to accept your responsibility or do I have to contact the police and tell them what you told me?" later adding: "It's not about her its about your accepting your responsibility to me."
After days of legal argument, Mr Justice Sweeney ruled against both applications a week ago, meaning legal action could proceed.
On Monday, Mr Kelsey-Fry invited the court to read the indictment to Huhne once more, even though he had already pleaded not guilty at last week's hearing.
When the allegation of perverting the course of justice was read he quietly declared that he was guilty.
Ms Pryce was also sitting in the dock and neither showed any emotion as he entered his plea.
Granting Huhne unconditional bail until a sentence date to be fixed, Mr Justice Sweeney said: "As Mr Kelsey-Fry has foreshadowed, you should have no illusions whatsoever as to the sort of sentence that you are likely to receive."
Ms Pryce's case was adjourned until 10:00 GMT on Tuesday, when a trial is expected to start.
Huhne, from Eastleigh in Hampshire, resigned as energy and climate change secretary after he was charged and has now forced a by-election in his constituency.
Nigel Farage, the leader of the UK Independence Party, said he would announce within the next 24 to 48 hours whether he would stand in the Eastleigh by-election. He previously ran for the same seat in a 1994 by-election.
"I haven't decided yet," he told Channel 4 News. "It's a bit of a shock, things happening today as quickly as they did, I am thinking very hard about whether to stand."
Mr Farage also said he had duties as an MEP in Brussels and that the party had to contest the forthcoming council elections.
"I've got to weight up my responsibilities," he said, adding that there were "many other good people in Ukip".
Huhne received a severance payment of about £17,000 when he resigned his cabinet position. Downing Street have said it was his decision whether to return the money.
The Liberal Democrats have said Huhne intends to voluntarily remove himself from the Privy Council, meaning he will lose his "Right Honourable" title.
Lib Dem leader of Eastleigh Borough Council Keith House said Huhne's resignation was "very, very sad," and described him as a hardworking MP.