The report underlines the growing pressure facing Prime Minister David Cameron as he contends with the rise of the eurosceptic UK Independence Party (UKIP) ahead of a May 2015 general election.
Cameron has vowed to renegotiate a return of powers from Brussels to London and hold a referendum on EU membership in 2017, and has said he would campaign for Britain to remain in the bloc.
But The Independent reported that "up to 100" MPS from his ruling Conservative party are preparing to defy their leader by pledging support for an EU exit in their 2015 election manifestos, regardless of any changes to Britain's membership.
Conservative MP Jacob Rees-Mogg said he had not been approached about the idea but said he was "very interested".
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"If the Prime Minister hasn't, by the election, set out very clearly his terms for renegotiation and the circumstances under which he would call for a No vote it would be extremely tempting to put a personal statement in one's election address to say that I will vote for Out," Rees-Mogg told BBC radio.
On Thursday the prime minister suffered a blow when Conservative lawmaker Douglas Carswell defected to UKIP, triggering a by-election that could make him the party's first elected member in the House of Commons.
Yesterday, another Conservative lawmaker, Chris Kelly, announced he would stand down at the 2015 election after serving one term. He did not give a reason for the decision.
Polish Prime Minister Donald Tusk, named Saturday as the next European Union president, said he could not imagine the bloc without Britain and pledged to do everything possible to meet London's demands for reform.