Chris Huhne's chances of keeping his Cabinet job were further undermined when Liberal Democrat colleagues began for the first time to openly question whether he should stay on.
The Energy Secretary is due to be interviewed this week by Essex Police over allegations that he persuaded his ex-wife, Vicky Pryce, to accept speeding penalty points on his behalf in order to escape a driving ban.
Pictures of her driving licence were made public yesterday. They showed a single, three-point fixed penalty offence for speeding on 12 March 2003 – the date Mr Huhne's car is alleged to have been caught by a speed camera.
Mr Huhne, who was a member of the European Parliament at the time, has denied claims that he was actually driving and would have been banned from driving unless Ms Pryce took the points for the offence.
The emergence of the driving licence adds to the suspicion that those who are trying to undermine Mr Huhne have the full support of his former wife.
It is alleged that Mr Huhne was returning from Stansted Airport to his home in Clapham, south London, having flown in from Strasbourg where the European Parliament sits.
Ms Pryce's licence is marked with the court code 1629, indicating that the offence was dealt with by Southend Magistrates' Court in Essex which covers the road network between Stansted and London.
The licence does not show the time of day the offence occurred. However, Ms Pryce is known to have spent much of the day in central London, attending a conference in the morning and an evening event at the London School of Economics where she stayed for a dinner until after 10pm.
Mr Huhne received less than enthusiastic backing from the Foreign Secretary, William Hague, who said it was up to him to decide whether he could remain in his post. Asked whether Mr Huhne should stand down, Mr Hague replied: "In the media, those things are being questioned, but the police are looking into it and so what can we do? We have to let that process take place. It's his decision."
There was also grassroots concern within the Liberal Democrats about Mr Huhne's position. Simon McGrath, who runs a popular Lib Dem Facebook page, said he should now stand down or be sacked. "However good Huhne is as Energy Secretary, it will be overshadowed by the continuing coverage of his private life," he wrote on the activist website LibDem Voice.
"If he chooses not to do this then Clegg should show that he will put the interests of the Party first and fire him."
The Sunday Times reported that Mr Huhne would tell police that he could "not be sure" whether he was driving the car on the day in question because it was such a long time ago.
However, this was denied by Mr Huhne's lawyer who said he had been "misrepresented" by the paper and that he had made "no admission" of anything. His "total unlimited denial" remained in place, she added.
Two newspapers published details last week of what was said to be a taped telephone conversation between Mr Huhne and Ms Pryce – who divorced earlier this year after he left her for another woman – apparently discussing the case.
Mr Huhne was reportedly urging her not to talk to journalists about the allegations, saying that there was no evidence to support the story "unless you give it some legs by saying something".
Shadow Cabinet Office minister Tessa Jowell said that David Cameron should set up an independent investigation to establish what had happened.
"That is what the Prime Minister should be doing," she said. "He should be getting a proper investigation independently to establish what the facts were and in the light of the facts decide the best course of action."