Nick Clegg has suggested that as he hardened the Liberal Democrats' commitment to giving the public a say.
Standing in for David Cameron at Prime Minister's Questions, Mr Clegg surprised MPs by saying: "It is a question of when, not if, because the rules [of the EU] are bound to change."
At the 2010 election, the Lib Dems promised an in/out referendum if there were a significant change in the relationship between the UK and the EU. Since then, the Coalition has brought in a law guaranteeing a referendum in the event of a significant transfer of power from Britain to Brussels.
Mr Clegg said: "We should have a referendum on Europe when the rules change. We said that at the time of the Lisbon Treaty. We said that in our manifesto. We have legislated on it and we will say it again."
The Lib Dems will not support Conservative plans to bring in a law before the 2015 election promising an in/out referendum by 2017, as David Cameron has promised. They argue that it would be wrong to commit to a referendum before negotiations on a new settlement with Europe have even begun. But Mr Clegg's statement is a sign that he does not want his party to be portrayed by the Tories as opposed to giving the public a say on Europe. It could put pressure on Labour to follow suit in its 2015 election manifesto.
In the Commons, Mr Clegg was taunted by Tory MPs, some of whom brandished a 2008 Lib Dem leaflet in which he declared: "It's time for a real referendum on Europe" at the time of the Lisbon Treaty negotiations.
One Tory, Edward Leigh, asked Mr Clegg whether the man pictured in the leaflet was "an impostor or just a hypocrite". Andrew Turner said his Isle of Wight constituents would feel "betrayed" if the Lib Dems did not support an amendment to the Queen's Speech, being debated today, regretting the absence of an EU referendum. It has been backed by 92 MPs, mostly Conservatives.
The Deputy Prime Minister hit back at his Tory critics, accusing them of "constantly shifting the goalposts" on a referendum. He said the Commons had spent 100 hours debating the Bill which gave a legal guarantee of a referendum if powers were transferred to Brussels. "We on this side should go out and promote what is in the Queen's Speech, not spending days bemoaning what is not in the Queen's Speech," said Mr Clegg. "I think we should stick to the priorities of the British people, which is growth and jobs."
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