Former cabinet minister Chris Huhne could face up to 18 months in prison after pleading guilty to perverting the course of justice.
The MP for Eastleigh admitted that his ex-wife Vicky Pryce, who still lives in the marital home in Clapham, took speeding points for him ten years ago.
The 58-year-old initially denied the allegation but changed his plea at Southwark Crown Court this morning.
Ms Pryce, 60, pleaded not guilty to the same offence on the grounds of marital coercion.
The original offence took place in March 2003 when Huhne's car was allegedly caught by a speed camera on the motorway between London and Stansted Airport.
It is alleged that between March and May 2003, Ms Pryce, 60, falsely informed police that she was the driver of the car so Huhne could avoid prosecution.
Speaking outside the court, Mr Huhne said: "I pleaded guilty today. I am unable to say more while there an outstanding trial.
"But 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."
His resignation will now trigger a forced by-election in his constituency in Eastleigh, in Hampshire.
Official sentencing guidelines on the Crown Prosecution Service's website state that concealing evidence, under a charge of perverting the course of justice, is punishable with a prison sentence of between four months and 18 months.
The length of the sentence could increase because Mr Huhne, who initially vowed to fight the charge, did not plead guilty at the first opportunity.
Carina Trimingham, whom the MP admitted having an affair with in 2010, was sat in the public gallery when he appeared in court.
Putney resident and LibDem leader Nick Clegg said he was "shocked and saddened" by his colleague's resignation.