Ed Miliband has drawn battle lines with the Tories over Europe by effectively ruling out a referendum on British membership - unless there are moves to transfer fresh powers to Brussels.
The Labour leader insisted he was determined to reform the EU if elected in 2015, and pledged a legal lock that would guarantee an in-out vote if Westminster was asked to hand over more control.
But he made clear that he believed that was "unlikely" to happen in the next parliament.
The comments, in an article for the Financial Times, come ahead of a speech on the economy he is due to make tomorrow.
Mr Miliband acknowledged that people had "concerns" about the EU and said Labour's "agenda for change" would seek to return "more control to national parliaments".
"It is important to emphasise that there are no current proposals - from either the EU or any member state - for a further transfer of powers from Britain," he said.
"Therefore it is unlikely there will be any such proposals in the next parliament.
"But the British people know that the history of the EU, as well as uncertainty about precisely what a changing Europe and further integration in the eurozone might involve, means that a further transfer of powers remains possible.
"So in a speech today I am announcing that the next Labour government will legislate for a new lock: there would be no transfer of powers from the UK to the EU without a referendum on our continued membership of the EU.
"This would not just be a referendum on the narrow question of whether to allow a transfer of powers from Britain to Brussels; as we have seen in other countries, such votes are too easily ignored. This position, setting out the conditions in the next parliament under which a Labour government would hold an in/out referendum, offers the British people a clear choice at the next election."