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Ed Miliband risks splitting the Labour Party unless he offers an in-out referendum on membership of the EU, MPs will warn today.

The Labour for a Referendum campaign says the Labour leader can no longer “ignore core supporters” by blocking an in-out poll on Europe.

The new group launched today is being bankrolled by millionaire donor John Mills and has the support of dozens of MPs, peers and more than 150 Labour councillors.

Mr Miliband has so far refused to commit to a referendum saying it could cause uncertainty for British business.

But supporters of Labour for a Referendum say Labour risks seeping support to UKIP and the Tories and could lose the next election unless it backs an in-out poll.

Labour MP Jim Dowd, a former whip, says a referendum would “unite the party.”

“I have been a supporter of this cause for many years and firmly believe the Labour Party must commit to a referendum before the European elections next year.

“As the Tories tear themselves apart over this issue, Labour for a Referendum provides the opportunity to unite the party on giving the people a say on our future in the EU,” he said

The campaign is also backed by One Foot in the Grave star Richard Wilson, who said: “Any sensible Labour Party member should support Labour for a Referendum, as a pro-European I want to put the issue to bed once and for all.”

Labour MP Graham Stringer added: “2010 was a low point for Labour support, I remember the 1983 election, the last time we got so few votes, and before that it was 1918. We should not and cannot afford to ignore our core supporters.”

Mr Mills, the chairman of Business for Britain, has donated £1.65million in shares to the party in the last year.

He is the brother-in-law of former Labour minister Dame Tessa Jowell and the founder of JML, an online shopping and retail company.

He said: “Labour for a Referendum will be at the heart of the discussion about our EU policy over the coming year.

"Labour MPs, councillors and activists from across the country recognise that fighting an election without a commitment to an EU referendum is dangerous.

"The Tories will paint us as the anti democratic party in 2015, if we don’t give the people a say.”

Mr Cameron has pledged to hold a referendum in 2017 if the Conservatives win the 2015 general election.

He is under pressure from Tory MPs to bring forward the poll before the election amid fears the Tories will be trounced by UKIP in next year’s European elections.