ED MILIBAND risks losing next year's General Election if he does not offer voters a referendum on the EU, the leader of the UK's largest union has warned.

Len McCluskey,the general secretary of Unite, which has given millions of pounds to the Labour party, warned the issue could be a deciding factor in a tight contest between Labour and the Conservatives.

Ahead of the Trades Union Congress (TUC), which starts in Liverpool this weekend, he said his union was concerned "that in a General Election where Ukip will be saying that everything that is wrong in the world is because of Europe and the Tories are offering a referendum we think for Labour not to offer a referendum [on Europe] may put Ed in a difficult position".

He added: "He is going to be portrayed as somebody who is afraid of asking the British people their views, and we think that is tactically dangerous for him."

Asked if the tactic might lose him the election, he said: "In a tight election, it could do, that is exactly right. That is our view. Which is why we would prefer to take a position of saying 'let's call a referendum' and then positively argue the values [of staying in the EU]." He added, however, that he did not think Mr Miliband would match David Cameron and offer an in/out referendum on EU membership in the next parliament.

He also attacked the eurosceptic Ukip, which has faced attacks on its immigration policies.

Mr McCluskey said: "For me this is a dangerous organisation for the simple reason that they give respectability to prejudice and bigotry."

He predicted that if there was an EU referendum, the UK would vote to stay in, after the case was made by a loose coalition inclu-ding Labour, the Conservatives and the LibDems, the trade unions, the CBI and business.

But he added: "I think there will be a comfortable Yes vote, I would not say a landslide."

Mr McCluskey also urged Labour not to enter a coalition with another party if it is the largest party after the next election but without a majority in the House of Commons.

The union has healed many of the rifts with the party since the row over allegations of vote rigging in Falkirk. At the time Mr Miliband accused Unite of wrongdoing amid claims the union was trying to stitch up the nomination for Labour's General Election candidate. Since then Labour has re-organised the rules around its relationship with the unions, reforms that have been warmly welcomed by Mr McCluskey,

At the TUC this weekend he said his message would be one of "hope" to working people, that someone was on their side arguing for wage rises and an ease to the squeeze on living standards.

Frances O'Grady, the general secretary of the TUC, has said she will urge the governor of the Bank of England, to keep interest rates low. Mark Carney is to be the first central bank boss to speak to the TUC since Sir Mervyn King in 2010.

The TUC has also warned that too many people are "under-employed" - in work but unable to do as many hours as they would like, a problem masked by the official unemployment figures.

Last night Grant Shapps, the Conservative party chairman, said: "Even Labour's largest donor thinks Ed Miliband is too weak to trust the people.

"Only the Conservatives will let Britain decide."