By Peter Griffiths
LONDON (Reuters) - Prime Minister David Cameron faces a humiliating challenge to his authority on Wednesday when scores of rebels in his ruling party will attack him in parliament over his stance on Britain's membership of the European Union.
In a further blow to Cameron's leadership, up to 100 Conservative lawmakers will take 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 is non-binding, a big mutiny would be an embarrassment for Cameron less than two years before the next parliamentary election and would embolden eurosceptics pushing him to take a harder line on Europe.
Cameron, who will miss the showdown because he is visiting the United States, played down its significance, saying he was "extremely relaxed" about what is a free vote for Conservative lawmakers, except ministers.
"I keep reading about the number of rebels. I hope someone will define for me how you can rebel on a free vote," he said.
The opposition Labour Party, which has a 10-point opinion poll lead, said the revolt showed Cameron was "panicked" over Europe and had lost control of his party.
"We have a prime minister who is not just indecisive, not just weak, but fast becoming a laughing stock," Labour deputy leader Harriet Harman told parliament.
Cameron had hoped to end party squabbling over Europe in January when he promised to renegotiate Britain's EU role and hold a referendum on its membership before the end of 2017, provided he wins the next general election in 2015.
But his gamble failed when Conservative eurosceptics soon began pushing for a law before 2015 guaranteeing the referendum will 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, to be held after a debate that ends at 1800 GMT, underscores 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 a powerful "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.
Cameron must balance the hardening anti-EU mood at home with the need to keep good ties with Britain's biggest trading partner. His pledge to claw back powers from Brussels angered France and Germany, who cautioned him against "cherry picking".
Eurosceptic Conservative lawmaker Bill Cash said Cameron should call a referendum before voters go to the polls in 2015.
"If we don't do that, we'll never have a referendum and we'll lose the general election," he told Reuters.
(Additional reporting by William James in London and Andrew Osborn in New York; Editing by Guy Faulconbridge and Giles Elgood)