Ed Miliband has reaffirmed his belief that Britain's future "lies in the EU" as he made clear a future Labour government is unlikely to stage a referendum on the UK's membership in the next parliament.
The Labour leader sought to end the uncertainty over his position, announcing that he would only hold a referendum if there was an attempt to transfer powers to Brussels - something which he said was not on the cards at the moment.
His announcement angered some Labour Eurosceptics, who have been pressing for the party to commit to an in/out referendum, while the Tories said his plan made "no sense whatsoever".
In a keynote speech, Mr Miliband confirmed that a future Labour government would legislate to require any transfer of power to Brussels to be put to the country in an in/out referendum on Britain's membership.
Speaking at the London Business School, he said that while that appeared unlikely to happen in the next parliament, his "legislative lock" should reassure a sceptical public that they would be given a say before it did.
"Now, there are no current proposals from other countries for such a transfer of powers. And I do not believe this is the direction in which Europe should head. Indeed, I think some powers should come back the other way. But we cannot be certain about the emerging shape of Europe," he said.
"So today I am announcing that the next Labour government will legislate for a new lock. Not simply a referendum on any treaty change proposing a transfer of power, b ecause there have been too many referenda like that in other countries which have been ignored, b ut a lock that guarantees that there will be no transfer of powers without an in/out referendum, w ithout a clear choice about whether Britain stays in the EU."
David Cameron - who is committed to re-negotiating the terms of Britain's membership and then staging an in/out referendum by 2017 - dismissed the Labour leader's promise, saying the only way the public could be sure of having their say on Europe was to vote Conservative.
"It is not a proposal for an in/out referendum. It is a policy clearly designed by a committee who obviously couldn't agree what to do and have come up with a policy that makes no sense whatsoever," the Prime Minister told reporters on his first official visit to Israel.
"The British people now have a very, very clear choice at the next election. If they get a Labour government, they will get no referendum, no choice, no reform, nothing. It is the classic Labour 'we know best' approach to politics."
Mr Miliband's plan also upset Labour Eurosceptics, with backbencher Graham Stringer denouncing it a "shoddy compromise".
"I think the public are very clear that they want a referendum. This is so ambiguous as to be impossible to sell on the doorstep."
Party donor John Mills, the JML home shopping entrepreneur and chair of Labour for a Referendum, warned the public would feel "short-changed" if they were not given a vote.
"I want to see a Labour government in 2015 and, as the party that trusts the people, I think we should recognise that the growth of the EU's influence over Britain in the past 40 years warrants a referendum regardless of future events," he said.
The reaction from business was more positive. CBI president Sir Mike Rake said: "Business will welcome Labour's decision to make its policy position on Europe clear. Any uncertainty is unhelpful when trying to secure long-term investment.
"The CBI strongly supports Ed Miliband's view that we are better off in a reformed EU than outside with no influence."
Terry Scuoler, chief executive of the EEF, the manufacturers' organisation, said: "Anything that reduces uncertainty about our membership of the EU will be welcomed by most businesses."
The legal lock proposed by Mr Miliband would effectively upgrade the one introduced by the coalition which guarantees a referendum before any powers are transferred.
Labour's new position aligns it very closely with the Liberal Democrats on Europe, offering common ground for any potential negotiations about forming a joint government in 2015.
Shadow chancellor Ed Balls denied suggestions that the announcement had left voters confused about whether the party would give a referendum or not.
He told LBC 97.3: "Every time you make an important announcement, and this is an important announcement from Ed Miliband, it always takes a day or so for people to decide what they really want to think about it, and you are always going to have some newspaper that have an agenda one way or the other.
"David Cameron has been saying today, and clearly some Conservative newspapers want to say, this means Ed Miliband ruling out a referendum. He's clearly not doing that."
He added: "The Labour Party has resolved that we want to be a pragmatic and hard-headed pro-European party."