British Prime Minister David Cameron has been challenged by a group of over 50 members of parliament from his Conservative Party who are prepared to join a campaign backing Britain's exit from the European Union unless he achieves radical changes in the bloc.

The MPs on Sunday launched a new political group called Conservatives for Britain (CfB), which will support Cameron's bid for reform while urging an end to EU membership unless significant changes are achieved.

Cameron is attempting to persuade European leaders to back UK demands for reform before holding an in-out referendum on Britain's EU membership. He has promised the vote by the end of 2017.

"We wish David Cameron every success, but unless senior EU officials awake to the possibility that one of the EU's largest members is serious about a fundamental change in our relationship, our recommendation to British voters seems likely to be exit," Conservative lawmaker Steve Baker said.

Cameron wants to restrict EU migrants' access to British welfare, improve the single market, and win safeguards to ensure countries outside the euro zone are not put at a disadvantage by greater integration.

Foreign Secretary Philip Hammond on Sunday said the UK government was seeking treaty change to protect its reforms, and that he believed the renegotiation could fix Britain's relationship with the EU.

"There was always going to be a group of our colleagues who wanted to come out of the European Union come what may," Hammond said when asked about CfB.

"That is not where the government is...we're clear that the European Union isn't working as it is at the moment, it's not fit for the 21st century, but we think it's fixable."

Baker said CfB had already signed more than 50 MPs and expected numbers to soon rise to about 100, including some ministers.

"I have been struck by the dozens of Tory (Conservative) MPs who would vote to quit the EU now and who will not settle for anything less than fundamental change," Baker, who is chairing the CfB group in parliament, wrote in the Sunday Telegraph.

The group said it would set out details of exactly what changes it wanted at a later date, but highlighted freedom-of-movement rules and repatriating lawmaking powers from Brussels as likely priorities.

On Saturday, the leader of the anti-EU UK Independence Party (UKIP) called for Eurosceptic politicians from rival parties to put aside "personal animosities" and begin the "No" campaign against Britain's continued membership of the bloc.

A survey by polling firm ICM indicated that 59 percent of Britons supported staying in the EU, and 41 wanted to leave, the Telegraph said.

In an interview with the Observer newspaper, Rafal Trzaskowski, Poland's Minister for European Affairs, said European leaders wanted Britain to stay in the EU, but not at any cost.

"Many people in Europe want to be accommodating," he said, "...but if the demands are too extreme, they are not going to be met."