Cameron confronted by 100 Tory MPs over 'broken promises' on Europe as Germany and France snub review of Brussels powers More than 100 Conservative backbenchers have written to David Cameron demanding legislation guaranteeing a referendum on Britain's future in Europe after 2015. The Prime Minister, who pledged earlier this year that he would offer voters an in/out referendum if he wins the next election, is under increasing pressure from MPs to put forward an "enabling" Bill in this Parliament. Several ministers and junior members of the Government are thought to be backing the idea, although only backbench MPs signed this week's letter. Downing Street insists it would be impossible to introduce a Government Bill, as the Liberal Democrats will not back a referendum. But eurosceptic Tories say a Commons vote on the issue before the general election in 2015 is vital to prove the party means to honour its pledge - and embarrass Ed Miliband and Nick Clegg, who oppose the idea. John Baron, the MP who organised the letter, said: "We are going to be looking for all opportunities to raise this on the floor of the House. Over 100 Conservative colleagues and I have urged him to bring the enabling legislation forward in this Parliament. This would address the lack of public trust. When people hear politicians making promises about an EU referendum, they don't know whether to believe them. The legislation offers the chance to convince the electorate. The referendum promise has to be credible and believable. Mr Cameron sent shockwaves through Europe when he gave a guarantee of an in/out vote on our membership if he is Prime Minister after 2015. He vowed to campaign for Britain to stay in - as long as Brussels agreed to renegotiate a looser, more trade-based relationship and return key powers to Westminster. And he has insisted that a referendum held by the end of 2017 would be a deal-breaker in any future coalition deal with the Liberal Democrats. A Downing Street source said: "There aren't any plans for legislation this side of the election, as opposed to afterwards, since it's quite difficult to introduce a Bill if one part of the Government doesn't support it. We have said that it is a red line for David Cameron. If he is Prime Minister after the next election, there will be a referendum in the next Parliament.