David Cameron is under mounting pressure from Conservative MPs to abandon plans to “opt in” to European Union police and justice measures, including the European arrest warrant.
Tory whips have warned the Prime Minister that between 80 and 100 of the party’s MPs could vote against the moves, forcing him to rely on Labour’s support.
It would be the biggest Tory revolt over Europe since the last general election.
Eurosceptic Conservatives are increasingly confident that Mr Cameron will perform a U-turn and drop the plans to sign up to 35 European Union measures.
Such a retreat would provoke a major rift with Theresa May, the Home Secretary, who backs the police’s view that the European arrest warrant will help them bring serious criminals to justice by enabling their speedy return without lengthy extradition procedures.
One Tory MP said: “We think Cameron will go our way on this and over-rule Theresa May. It would be very hard for him to justify such a big step towards EU integration when he is trying to get our voters back from Ukip. They would not understand it.”
Another said: “We are detecting sympathy on this from several ministers.”
Previous estimates had put the likely Tory rebellion at about 40. But the rebels say their ranks have swollen in recent weeks as the threat from Ukip to the Tories’ general election prospects has grown.
However, Cameron aides insisted last night there had been “no change of policy” and dismissed the Eurosceptics’ claims that one is in the pipeline. Home Office sources also stressed that Ms May remained determined to opt in to the European arrest warrant, believing it essential for tackling terrorism and paedophile networks.
As the Conservative whips gauge MPs’ opinion on the subject, Ms May’s allies insisted some of the waverers will come back into line.
They pointed to the Home Office’s problems in removing foreign prisoners, which were revealed this week in a damning National Audit Office report, as evidence of the need for Britain to remain in the warrant system.
“When it comes to the crunch, I think some of them will be more driven by their dislike of foreign criminals than by their dislike of Europe,” said one source.
The Government has decided to opt out of more than 130 European criminal justice measures on December 1 and will then immediately opt back in to 35 of them which it believes are beneficial to Britain.
As well as the arrest warrant, the list includes signing up to Europol, the agency which shares intelligence between police forces across the European Union.
Ministers have committed to holding a vote on the changes before December 1, which has been provisionally pencilled in for the last week of November.
The Liberal Democrats would oppose a U-turn by the Prime Minister, and Labour remains committed to the retention of the warrant.
Eurosceptic critics argue that the warrant has been unfairly used to force the extradition of UK nationals on minor and insubstantial charges.