By Estelle Shirbon

LONDON (Reuters) - Former British energy secretary Chris Huhne faces time behind bars after pleading guilty on Monday to perverting the course of justice by asking his then wife in 2003 to accept a penalty for a speeding offence he had committed.

A senior member of the Liberal Democrats, the smaller party in Britain's ruling coalition, Huhne, 58, entered his guilty plea on the morning his trial had been due to start at London's Southwark Crown Court.

It was a dramatic change of tack after he had spent months fighting a costly legal battle to have the charge against him dropped. After losing that battle a week ago, Huhne had initially pleaded not guilty to the offence.

Huhne had resigned from his cabinet post when he was charged in February 2012. Shortly after Monday's plea, he said on the steps of the courthouse that he would now also leave parliament.

"Having taken responsibility for something which happened 10 years ago the only proper course of action for me is now to resign my Eastleigh seat in parliament," he told reporters.

Deputy Prime Minister Nick Clegg, who narrowly defeated Huhne to become the Liberal Democrat leader in 2007, said he was "shocked and saddened" by Huhne's guilty plea.

Huhne had still been seen by some members as a possible successor to Clegg until Monday's news.

Huhne had been supposed to face trial before a jury alongside his estranged ex-wife, economist Vicky Pryce, who pleaded not guilty last year to a charge of perverting the course of justice connected to the same 2003 speeding offence.

Following Huhne's guilty plea, Pryce, 60, a former joint head of the government economic service, is now scheduled to face trial alone starting on Tuesday. Her lawyers have indicated that her defense would be that she did accept the penalty for Huhne's speeding offence to help him avoid a driving ban but was not to blame because there had been "marital coercion".

Huhne had already accumulated penalty points on his driving license for three previous speeding offences, and, if he had accepted blame for the March 2003 incident, he would have been banned from driving for a period.

However, despite avoiding the penalty then, Huhne did lose his license for a time after police caught him later the same month talking on his mobile phone while driving.

Allegations that Pryce had taken penalty points on Huhne's behalf in 2003 first surfaced in newspapers in 2011, months after Huhne left Pryce for his mistress, public relations adviser Carina Trimingham, ending a 26-year marriage.

While Huhne and Pryce were in the dock together during Monday's proceedings, Trimingham sat just behind them in the public seats.

FAMILY BREAKDOWN

Huhne remains free on unconditional bail pending his sentencing at an unspecified later date.

"You should have no illusions whatsoever as to the sort of sentence that you are likely to receive," Judge Nigel Sweeney told Huhne after the guilty plea.

"Understood?" the judge asked Huhne, who nodded from the glass-walled dock.

The maximum sentence for perverting the course of justice is life in prison but that is very rarely applied. The average sentence for the offence is about 10 months.

As recently as last month, Clegg had described Huhne as "a big beast" and told reporters he would like to see to see the ex-minister return to the "top table" of British politics, were he cleared of the charge.

Huhne's failed application to have the charge against him dismissed, which was heard last month, gave a flavor of the intimate details that would emerge as evidence during the trial.

In particular, the court heard that the prosecution would present transcripts of text messages between Huhne and his son Peter, the youngest of his three children with Pryce. The prosecution said the texts were relevant because they contained an exchange about the alleged 2003 offence.

The texts, from 2010-2011, gave a painful insight into the breakdown of the family. In them, Huhne repeatedly tried to reach out to the then 18-year-old only to be rebuffed with insults and expletives.

The court heard that Peter Huhne had called his father "the most ghastly man I have ever known" and had repeatedly told him to stop contacting him.

Media had not previously been able to publish the texts because of reporting restrictions on pre-trial proceedings designed to avoid prejudicing jurors in the trial. Some of those restrictions were lifted after Huhne's guilty plea.

(Editing by Alison Williams)