Union threatens to turn off the taps for Miliband

GMB will vote on whether to end link with Labour in protest at party's stance on public sector pay

Ed Miliband is facing the threat of an open break with one of Labour's biggest union backers because of his refusal to promise to end the public pay freeze.

Delegates to the annual conference of Britain's third biggest union, the GMB general union, will hold a vote on whether to put an end to the formal link with the Labour Party – a move that would deal a body blow to the party's already flimsy financial base.

While Mr Miliband may not be too unhappy at being attacked by union activists as he tries to win back middle-class voters who deserted Labour in 2010, losing the financial backing of the GMB would be a serious blow. The 600,000-strong union gives the Labour Party more than £2m a year.

The mood of nervousness among Labour MPs who value the union link was heightened yesterday when they received invitations to a Commons meeting with Len McCluskey, the left-wing head of Unite, the biggest union of all.

Mr McCluskey who is also critical of Mr Miliband's leadership, has booked one of the biggest rooms in Parliament next Wednesday and invited all 258 Labour MPs to come and hear what he has to say, provoking fears that he will try to use the union's financial clout to alter party policy.

But a union spokesman insisted: "It's a regular meeting. We have regular meetings with Labour MPs. We'll talk about the Health Bill and other issues. This is not connected to what's happening at the GMB."

The GMB's ruling executive met yesterday to consider the resolutions from union branches around the country for debate at their annual conference . Most were passed in January, to meet a 31 January deadline – but inside sources say there was a notable difference in tone between those passed in the first two weeks of the month, and those that came later, which were more critical of the Labour leadership.

This change of tone is attributed to Mr Miliband's appearance on the BBC on 15 January, which was followed by an announcement by the shadow Chancellor, Ed Balls. Both men warned that they could not promise that a future Labour government would reverse tax rises or spending cuts imposed by the Coalition. The comments have been interpreted by several Labour-controlled councils around the country as their cue to freeze staff pay for another year.

The GMB has 240,000 members directly employed by the public sector, and 60,000 working for private firms contracted by the public sector.

A statement yesterday from the GMB executive said: "The union's relations with the Labour Party and what GMB members expect and want from the party will form a major plank in the debate at GMB Congress in Brighton in June."

The statement also "expressed concern and disappointment with recent statements made by senior party officials and registered their growing frustration at the lack of a cohesive policy to protect working people from the ravages of the Tory-led Coalition Government."