LONDON (Reuters) - Prime Minister David Cameron suffered an embarrassing blow in parliament on Wednesday when more than a third of his Conservative MPs voted against him in protest at his stance on Britain's membership of the European Union.
Though the revolt was defeated, it could undermine Cameron's leadership, as scores of his own party's MPs took the highly unusual step of voting to criticise his government's legislative plans, a week after they were first put before parliament.
The rebels are angry that the government's policy proposals did not include steps to make Cameron's promise of a referendum on Britain's EU membership legally binding.
The party turmoil has fuelled talk of Britain sliding towards the EU exit and has stirred memories of Conservative infighting that contributed to the downfall of former prime ministers Margaret Thatcher and John Major.
While the vote was non-binding, the scale of the mutiny, less than two years before the next parliamentary election, will embolden eurosceptics pushing him to take a harder line on Europe.
A total of 130 MPs supported an amendment expressing regret that the EU referendum was left out of the government's agenda. Of those, 114 of the Conservatives' 305 members of parliament voted against Cameron.
Senior Conservatives put on a brave face after the largely symbolic ballot, stressing that they had allowed their members to vote freely on the issue, except for government ministers.
"This was a free vote," Foreign Secretary William Hague said. "When all the dust has settled, there is one essential fact: one party, the Conservative Party, is committed to a referendum on leaving or staying in the European Union, and the other parties are not."
Just before the parliamentary ballot, Cameron played down its significance, saying he was "extremely relaxed".
However, the opposition Labour Party said the revolt showed Cameron was losing control.
"This vote is a further devastating blow to the prime minister's authority," Labour foreign affairs spokesman Douglas Alexander said. "Cameron has managed to turn a Europe issue into a leadership issue."
Cameron had hoped to end party squabbling over Europe in January when he promised to renegotiate Britain's relationship with the EU and hold a referendum on its membership before the end of 2017, provided he wins the next general election in 2015.
But Conservative eurosceptics soon began pushing for a law before 2015 to guarantee the referendum would take place. Some even called for an earlier referendum.
Cameron's offer on Tuesday of draft legislation that would make his pledge legally binding received a lukewarm reception. Rebels say it will be blocked by the Conservatives' coalition partner, the pro-EU Lib Dems.
Wednesday's parliamentary vote underscored how Cameron is boxed in over Europe.
Keen to avoid a rift with the Lib Dems, he must also avoid alienating Conservative eurosceptics who see the EU as an over-mighty "superstate" that threatens Britain's sovereignty.
The success of the anti-EU UK Independence Party in local elections this month only intensified Conservative pressure for Cameron to go further on Europe. A YouGov poll in April put support for withdrawal at 43 percent, with 35 percent wanting to stay in.
Conservative lawmaker Nadine Dorries, who was punished by her party last November when she left parliament to appear on a reality television show, said she was considering standing as a joint Conservative-UK Independence Party candidate at the next election.
She said some Conservatives had a "huge amount of empathy with UKIP".
Cameron responded by saying "the party doesn't do pacts and deals".
Additional reporting by William James and Costas Pitas in London and Andrew Osborn in New York; Editing by Guy Faulconbridge and Will Waterman