The pressure grew on Chris Huhne last night after Essex Police disclosed it had launched a formal criminal investigation into claims he attempted to evade penalty points for speeding.
The Energy Secretary allegedly asked his estranged wife, Vicky Pryce, to accept penalty points on his behalf so he could avoid a driving ban. He has strongly denied the claim.
Both Mr Huhne and Ms Pryce are expected to be interviewed by police next week.
Essex Police said: "An investigation is ongoing in relation to the circumstances surrounding the issue of a fixed penalty notice in 2003. It would be inappropriate for police to discuss this matter any further in order to avoid prejudicing the investigation."
Previously the force had said an investigation would not be formally launched until its enquiries showed an offence had been committed.
Mr Huhne did, however, get backing of sorts from the Business Secretary, Vince Cable, who said it would be "a loss" to the Government if he were forced to resign from the cabinet because he is "very effective".