By Peter Griffiths
LONDON (Reuters) - Prime Minister David Cameron suffered an embarrassing blow in parliament on Wednesday when a third of his Conservative lawmakers voted against him in protest at his stance on Britain's membership of the European Union.
Though the revolt was defeated, the rebellion could undermine Cameron's leadership, as scores of his own party's lawmakers took the highly unusual step of voting to criticize 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.
Just before the vote, Cameron played down its significance, saying he was "extremely relaxed" about what was a free vote for Conservative lawmakers, except ministers.
"It's a free vote, and as I've said I'm relaxed about that, so I don't think people can read in anything to the scale of that free vote," he told reporters in New York, where he is on an official trip.
Labor deputy leader Harriet Harman said earlier that Cameron was "becoming a laughing stock".
A total of 130 lawmakers voted against the government. More than 100 of them were expected to be confirmed as Conservatives when the full voting figures are released later. The center-right party has 305 members of parliament.
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 Liberal Democrats.
Wednesday's parliamentary vote underscored how Cameron is boxed in over Europe.
Keen to avoid a rift with the Liberal Democrats, 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.
(Additional reporting by William James in London and Andrew Osborn in New York; Editing by Guy Faulconbridge and Will Waterman)