British Prime Minister David Cameron bowed to pressure from his restive Conservative party on Tuesday and published a bill to hold a referendum on membership of the European Union.
The move was designed to head off growing turmoil in the party the day before many Conservative MPs were expected to vote for a parliamentary motion condemning the coalition government's European policy.
It also came just hours after US President Barack Obama publicly backed Cameron's plan to renegotiate Britain's terms of membership with Brussels and put them to an in-out referendum by the end of 2017.
The Conservatives' draft bill would enshrine into law Cameron's commitment to the referendum plan, which is contingent on the party winning re-election in 2015.
But it is a highly unusual move because it has been published by the party, not the government, and will rely on an individual Tory MP guiding it through parliament as a private members' bill.
Foreign Secretary William Hague acknowledged the bill had almost no chance of becoming law, but said there could be no government legislation because of opposition from the Tories' junior coalition partners, the pro-European Liberal Democrats.
"But it means there can be a debate in the House of Commons on our policy, it means there can be a vote in the House of Commons," he told BBC radio.
"When all the dust settles on the speculation about this, one thing will be very clear... the Conservative Party is very much behind and committed to this policy."
Cameron has been under intense pressure over Europe in recent months, fuelled by the growth of the upstart UK Independence Party (UKIP) which is poaching Tory voters with its demands to pull Britain out of the EU.
The prime minister wants to try to reform the 27-nation bloc before holding a in-out referendum, in which he plans to vote to stay in, and has accused those urging an exit now of "throwing in the towel".
After talks in Washington on Monday, Obama offered his support for Cameron in an unusual intervention in British politics.
"David's basic point that you probably want to see if you can fix what's broken in a very important relationship before you break it off makes some sense to me," Obama said.
But calls for Britain to quit the EU have been getting louder, and two Conservative ministers said this weekend they would vote to exit the EU if a referendum were held now.
At least 67 Tory MPs have said they will support a parliamentary motion on Wednesday condemning the absence of a referendum law from last week's Queen's Speech, which set out the government's programme for the year.
The chief rebel, John Baron, said the draft bill was "not good enough" and would not affect the vote.
"By far the better option is for the government to have the courage to support our amendment on Wednesday," he said, although this would involve the bizarre situation of ministers voting against their own legislative programme.
Cameron's spokesman said he was "very happy" for the vote to go ahead, while repeating that ministers had been told to abstain.
He denied that the government's policy was in chaos, adding that the draft bill was "entirely consistent" with the prime minister's approach to the EU so far, and had been planned for some time.
But the Lib Dems appeared to have been taken by surprise by the move, which puts further distance between the coalition parties two years before the election.
The opposition Labour party accused Cameron of losing control.
"This seems to be just the latest panicked response from the prime minister who is now following, rather than leading his backbenchers," said senior Labour figure Douglas Alexander.
Labour opposes legislating now for a referendum in four years' time, although it too is divided over Europe.
A new Guardian/ICM poll found 35 percent of Britons favoured an immediate referendum, while 43 percent said they would vote to leave the EU and 40 percent said they wanted to stay in.
UKIP leader Nigel Farage, whose anti-immigration, anti-EU party secured one quarter of the vote in recent local elections, said Cameron's bill was "an act of sheer desperation".