David Cameron was dealt a "devastating blow" when more than 100 Conservative MPs backed a vote criticising the Government's failure to bring forward plans to enshrine an EU referendum in law.
A total of 114 Tory MPs, including 13 ministerial aides, voted for an amendment to the Queen's Speech expressing regret that Government had not included the plans in its new legislative programme, while two more acted as tellers for the vote, taking the total number of supporters to 116.
The Prime Minster insists nothing can be read into the result because he gave backbenchers a free vote, but opponents were quick to smell political blood, claiming the premier had "lost control of his party".
John Baron, the amendment's author, told Sky News: "The public don't trust politicians on this issue, that's why legislation is important when we promise an in/out referendum in 2017. That's what this battle is about."
Peter Bone, one of the MPs who triggered the vote, told the BBC: "This is not a rebellion, this is a free vote. In fact, to a certain extent, the Prime Minister was encouraging us to vote for the amendment because, after all, it's his own policy."
Despite the support, the amendment was defeated by 277 votes to 130, majority 147 but Labour claimed it left the PM badly bruised.
Shadow foreign secretary Douglas Alexander said: "This vote is a further devastating blow to the Prime Minister's authority.
"It demonstrates that David Cameron has managed to turn a Europe issue into a leadership issue.
"This is a Prime Minister who has lost control of the agenda and tonight lost control of his party. David Cameron's backbenchers have shown they simply won't give up until he gives in."
Former Lib Dem leader Sir Menzies Campbell mocked Mr Cameron. "I have done a lot of unusual things in the 26 years I have been a Member of Parliament but this is the first time I have voted for a Prime Minister to defend him from his own party," he said.