'Conservatives for Britain' pressure PM over EU talks
More than 50 Conservative MPs say they will vote to leave the EU unless David Cameron secures far-reaching changes to the UK's relationship with Europe.
The new group includes former ministers Owen Paterson and John Redwood.
It is calling for "fundamental" reform including restricting freedom of movement in the EU and making Westminster sovereign over EU law.
Foreign Secretary Philip Hammond said the government wanted the UK to remain in a reformed European Union.
Asked on the BBC's Marr Show about the MPs' group, called Conservatives for Britain, he said: "There was always going to be a group of our colleagues who wanted to come out of the European Union, come what may."
This was not the government's view, he added.
Mr Hammond also said he believed treaty change would be needed to secure the changes the PM wants to achieve ahead of the referendum, including restricting EU migrants' welfare entitlement.
These changes "can only be sustained against judicial attack" if they are backed by treaty change, he said.
The referendum will be held "as soon as we are ready" with the "back stop" date of 2017, he added, saying the EU could be "flexible" and make reforms if needed.
"They can do these things when they want to do them," he said.
Conservatives for Britain will style itself as a supporter of the prime minister's renegotiation plans.
Its co-chair, MEP David Campbell Bannerman told the BBC: "We are backing the PM. We are backing the government on renegotiation but we do want to see serious fundamental reform.
"Anything that smacks of tokenism or minor change is not saleable to the British electorate."
He denied the group was a "nascent out campaign", but its name echoes the title of another Eurosceptic group supporting a renegotiated settlement in Europe - Business for Britain.
Conservatives for Britain, which is run by MP Steve Baker in the House of Commons, has already met and plans further meetings in the week ahead.
It is hopeful of getting the support of more than 100 Conservative MPs.
Writing in The Sunday Telegraph, Mr Baker said: "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."
The MP for Wycombe added: "Crucially, we must end the supremacy of EU law over ever more matters of British life. That appears to be the majority view among Conservative members of Parliament.
"We need a Parliament which can decide the level of British taxpayers' contributions to the EU, what regulations should apply to our businesses, how to control EU migration and our trade relations with the rest of the world."
Former cabinet minister Owen Paterson said Eurosceptic ministers should be allowed to campaign for a No vote if they "are not happy with the deal".
"If that is not allowed, these people - if they have got any character about them at all and are interested in the future of their country - should stand down and campaign according to their conscience," he said.
Mr Cameron has opened talks with other European leaders including German Chancellor Angela Merkel and Polish Prime Minister Ewa Kopacz - who both want to ensure the UK stays in the EU.
Rafal Trzaskowski, Poland's secretary of state for European affairs, said those who would vote in the UK referendum must be told the truth about the consequences of leaving the EU.
He told the Observer: "If you say you can leave and still be part of the internal market and keep your second houses, that you will still be free to travel, that there will be no customs duties, and so on and so forth - but that you will not have to accept free movement of workers, and you will not pay into the EU budget, of course people will vote to leave, but this is simply not true," he said.
"You cannot keep all the goodies and forget about the costs. Britain will still have to pay into the EU budget, just as the Swiss and Norwegians do," he added.
"It would have no influence over the decisions yet it would have to subscribe to all the rules."
Mr Cameron is expected to set out more detailed demands for a new deal at a summit in Brussels later this month.
The new terms of Britain's membership will then be put to the public in a referendum before the end of 2017, asking whether Britain should remain in the EU.