Labour will consider calling for a referendum on whether Britain should leave the European Union, in its wide-ranging policy review.
An "in or out" vote has traditionally been demanded by Conservative Eurosceptics, But some pro-Europe Labour figures, who believe the public would vote to stay in, are promoting the idea as a way of settling the question for a generation.
The high-risk move opens up the possibility that all three main parties might support a referendum on Europe at the next election.
Conservative MPs are increasingly hostile to the European project after the dispute with the European Court of Human Rights over whether prisoners should have the right to vote and rapists and paedophiles be given the chance to have their name removed from the sex offenders' register.
The Liberal Democrats are pro-European but at last year's general election supported an "in or out" referendum the next time Britain signs up to a fundamental change to its relationship with the EU.
Labour's stance will be discussed today at a Fabian Society conference in London, "Britain and Europe: in, out or somewhere in between?" Sunder Katwala, general secretary of the Labour-affiliated society, said: "There is unlikely to be a referendum on British membership before 2015. But there must be a good chance of a referendum by the time of the 50th anniversary of the last referendum to stay in, in 1975, in order to settle the question of British membership."
He admitted that a plebiscite could create economic uncertainty, but said that the risks of losing the vote were not a good argument against holding one.
Wayne David, the shadow minister for Europe, said: "The Labour Party is having a fundamental policy review and this is one of the things that will be considered." He was "still to be convinced," he said, but some Labour figures believed a referendum would "lance the boil" of Euroscepticism.
Ed Miliband is thought to be wary about the idea. "This does not represent mainstream opinion," one ally said last night.