The time has come for a referendum on whether Britain should stay in the EU, Lib Dem leader Nick Clegg has said.
Mr Clegg said debate on the EU had been 'poisoned'
Politicians had "all gone crazy" over ratifying the Lisbon Treaty, taking attention from the "big question" on continuing EU membership, he added.
The Lib Dems are expected to table a House of Commons motion on Tuesday demanding a public vote on whether Britain stays in the EU.
But the Conservatives said the Lib Dems had "no mandate" for this.
At the last general election in 2005 all three main parties promised a referendum on the planned European Constitution.
But the constitution was discarded after it was rejected by voters in France and the Netherlands.
The EU Reform Treaty - signed by national leaders in Lisbon last December - is the replacement.
The Conservatives and the UK Independence Party are calling for a referendum on whether the UK should ratify the treaty, arguing it is largely the same as the constitution.
But Labour argues that the documents are very different and that Parliament should decide.
MPs are currently engaged in a 12-day debate on ratification.
The Lib Dems say they would abstain if the Conservatives successfully call for a referendum on the treaty - they say they would instead try to insert an amendment into the European Union (Amendment) Bill demanding one on the wider issue of whether Britain stays in the EU.
Mr Clegg, a former member of the European Parliament, said: "The debate over Britain's future in Europe has been poisoned for too long by a Labour Party that refuses to make the case for it, and an opportunistic Tory Party that actively seeks to undermine it.
"We need to draw the poison from that debate - to settle the matter one way or another.
"So today I am inviting the Labour and Conservative parties to join with me in calling for the referendum that will settle Britain's European future, once and for all: an in-out referendum."
Mr Clegg told the BBC that the planned referendum on the constitution had been "tantamount" to one on whether to continue EU membership.
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He described the Tory focus on the treaty as a "side-show" and a "distraction".
Instead, it was time to decide "are we going to carry on with this ambivalent attitude or withdraw" from Europe, or "make a whole-hearted long-term commitment".
Fore the Conservatives, shadow foreign secretary William Hague said: "This is an attempt to paper over the Lib Dems' widening split on a referendum on the renamed EU Constitution.
"But the key question remains whether the Liberal Democrats will honour their manifesto promise. The Lisbon Treaty is in substance, nearly identical to, the EU Constitution.
"That is what the Lib Dems promised their voters a referendum on. They did not mention an in-out referendum to voters - they have no mandate for one."
UK Independence Party leader Nigel Farage criticised Mr Clegg's proposal as "nothing but a smokescreen".
He said: "Whilst in the long term I agree that this is the referendum we want, calling for it at this time is only to cover up their weasel-like position over a referendum.
"Instead of hiding behind this call, they should be honouring the promise they made to their voters that they would support a referendum on this treaty."
Lib Dem justice spokesman David Heath has said he will defy his party's whips and back the Tories in calling for a referendum on the treaty.
Last week the European Parliament debated, and overwhelmingly backed, the treaty.
All 27 EU member states have to ratify it for it to come into effect.