DAVID Cameron is under mounting pressure from Conservative eurosceptics as the Home Secretary announced plans to opt out of a raft of EU criminal justice measures.

Theresa May said it was in the national interest for the UK not to sign up to more than 130 criminal justice measures, just a day after Education Secretary Michael Gove reportedly said: "We are ready to quit [the EU]."

Boris Johnson, the Conservative Mayor of London, became the latest senior Tory to call for a tougher line on Europe yesterday.

Mr Gove, a close confidant of the Prime Minister, is said to have told friends that if there was a referendum now on whether the UK should cut its links with Brussels, he would vote to leave.

He is said to be among up to eight Coalition ministers to support the withdrawal of the UK from the EU.

However, Downing Street has made it clear that Mr Cameron is committed to the UK remaining part of the EU.

Mr Johnson weighed into the debate to argue it would be misguided and wrong for the Government to support closer fiscal integration within Europe.

A source close to the Mayor said "the idea of closer economic and political integration is nonsensical.

"We need to resist all moves to further erode democracy in Europe and closer integration means just that, with Germany's position strengthened and everyone else's hopelessly weakened."

Last month, Mr Cameron suggested he would seek fresh consent for the role of the EU, but after the next election in 2015.

Mrs May said the Coalition Government would opt out of the criminal justice measures, which ministers can decide to opt back into individually at a later stage.

However, Liberal Democrats have already expressed concerns at the plans.

Mrs May told MPs: "The UK cannot pick and choose the measures from which we wish to opt out. We can only opt out en masse and then seek to rejoin individual measures."

The move was backed by Tory MPs. However, there was controversy over what it would mean for the European Arrest Warrant, the measure that saw teacher Jeremy Forrest returned to the UK after he allegedly disappeared with a teenage pupil.

Labour's Keith Vaz, chairman of the Home Affairs Select Committee and a former Europe minister, said the warrant started as a good idea but had "chaotic and unfair consequences".

But he urged Mrs May to look at opting-in to other measures on Europol and cross-border policing.

Shadow Home Secretary Yvette Cooper accused the Coalition of a "chaotic" position. Opting back in to justice measures could cause problems and cost taxpayers money, she warned.

She said: "There is no guarantee the European Commission and other European countries will actually support us opting back in again."

The Law Society of Scotland united with its counterparts in England, Wales and Northern Ireland, to call for a full and open consultation on whether the UK should opt out of the measures, including the European Arrest Warrant.

President Austin Lafferty said: "A wholesale opt-out could have very serious consequences in fighting cross-border crime from both a practical and cost perspective.

"Such a decision should not be taken before a thorough consideration of the implications, with input from the experts, who are certainly not all working in Government."

At the weekend, it was reported that Mr Gove gave other EU nations an ultimatum: "Give us back our sovereignty or we will walk out."

Tory Defence Secretary Phillip Hammond said in an interview that his alleged comments had reflected a hardening of attitudes among the Cabinet.