Over 100 Conservative backbenchers have again expressed their displeasure with the prime minister's position on Europe, in another setback for David Cameron's authority.
Eurosceptic John Baron delivered the letter to No 10 this morning, just days after Cameron sought to bolster party discipline by bringing a number of backbench Tory MPs into his Downing Street policy operation.
The letter calls for referendum legislation to be passed before the next general election ensuring the in-out referendum on Britain's ongoing membership of the European Union will be held in the next parliament.
Cameron promised in his Bloomberg speech in January that the in-out referendum would take place if the Conservatives are elected in 2015.
"Over 100 Conservative colleagues and I have urged the prime minister to bring the enabling legislation forward into this parliament," Baron said.
"This would address the lack of public trust when people hear politicians making promises about EU referenda – too many have been broken in the past.
"Legislation stands more chance of convincing the electorate than election promises."
Baron warned the Conservatives face a serious threat from Ukip. Rebutting the dismissals of some senior Tories, like Michael Ashcroft, that they are nothing more than a protest party, he said the Conservatives were more likely to win votes in Thursday's local elections if they had a "credible and believable" approach to Europe.
"The PM's commitment to the referendum has to be both credible and believable. It is credible because it has an 'out' option. But it is not yet believable," Baron warned.
"The PM should therefore bring forward the legislation into this parliament. Opposition splits would see it through but, whether successful or not, people would know he was serious. Why, then, both voting Ukip?"