More than 50 Tory MPs are in a new group that has pledged to fight to take Britain out of the EU if David Cameron fails to secure the reforms he is seeking.
The Conservatives for Britain (CfB) includes ex-cabinet ministers Owen Paterson and John Redwood and they have said they will support his efforts to renegotiate the UK's terms of memberships.
But the group has pledged to campaign for a 'no' vote in the in/out EU referendum Cameron has promised.
The group could be a headache for the PM, who has been accused by some, including Nigel Farage, of not wanting to leave the EU in any case of campaigning to stay in regardless of what he achieves.
Conservative MP Steve Baker, the campaign's Westminster chairman, said: "Conservatives for Britain supports the Prime Minister in seeking fundamental change in our relationship with the EU.
"The Conservative Government has promised the British people an in/out referendum on Britain's EU membership and we must explore the possibility of leaving if the EU do not agree to radical reform."
MPs on CfB who have agreed to be named are Bernard Jenkin and Julian Lewis as well as newly elected MPs James Cleverly, Tom Pursglove, Anne-Marie Trevelyan, and Craig Mackinlay, The Daily Telegraph reported.
Mr Paterson, a former environment secretary, told The Telegraph: "We all hope that the deal is satisfactory and will be widely accepted but if there are individuals in the Cabinet who are not happy with the deal, they should be allowed to campaign.
"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."
Rafal Trzaskowski, Poland's secretary of state for European affairs, said voters must be told the truth about the consequences of leaving the EU.
Asked whether he believes the UK might vote for exit, he told the Observer: "It really depends on how the whole thing is depicted by British political parties to the British people - what kind of alternative is painted.
"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 added: "If Great Britain leaves, it will minimise its role. I am not trying to be patronising. I think we have to be frank with the British people when we talk about their future in the European Union. Many people in Europe want to be accommodating ... but if the demands are too extreme, they are not going to be met.
"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.
"It would have no influence over the decisions yet it would have to subscribe to all the rules. London would not be that sexy a place for capital movement because it would have much looser links to Europe."