Nick Clegg will this week give backing from the Liberal Democrats for a referendum on membership of the European Union.
In a major speech on the future of Europe, the Deputy Prime Minister will say it is no longer a question of if but when the vote comes. Mr Clegg, a former MEP, will also warn Tory Eurosceptics that leaving the EU would be "economic suicide".
The announcement leaves Labour as the only party to not back an in-out referendum, with Ed Miliband appearing isolated on the issue.
In a speech on Tuesday, Mr Clegg will say: "We are no longer asking if Britain will have a referendum on continued membership; we are asking when Britain will have a referendum on continued membership.
"The parties differ on the timing: the Conservative Party wants one in 2017, regardless of what's happening in Europe at that time: it's a date chosen for internal party management as much as anything else. The Lib Dems believe it will be far better to have the referendum when a serious change to Europe's rules, affecting the UK, next arises."
In one of the most strident speeches on Europe that Mr Clegg has made, he will warn that the Tory plan to renegotiate powers back from Brussels ahead of a 2017 vote is "a seductive offer", but one that is "deeply flawed and bound to unravel", adding: "It collapses under the weight of its own internal contradictions.
"For many on the right, this... is just a smokescreen for exit. Politics masquerading as patriotism. It's a short-sighted political calculation that could jeopardise the long-term national interest. It is playing with fire and, if we go down this track, it is Britain that will get burnt."
Mr Miliband was urged to set out Labour's policy on a referendum in his conference speech in Brighton last month. However, the Shadow Cabinet is split over backing a vote. Despite pressure on the Labour leader to hint at a referendum, he made no mention of Europe. Labour sources said it would be wrong to commit now to a 2017 referendum as it would lead to economic uncertainty.
In a speech entitled "In Europe for the National Interest", Mr Clegg will add: "Our economy is finally turning a corner, but the recovery is fragile. We should be focusing on finishing the job and laying the foundations for long-term growth, not entertaining the idea of an EU exit that would throw our recovery away. Leaving the EU would be economic suicide. You cannot overstate the damage it would do to British livelihoods and prosperity."
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