One of the Labour's largest union donors is to debate its future links with the party, in the latest sign that relations between Ed Miliband and the unions are deteriorating.

The GMB said a quarter of motions to its annual conference in June related to the union's political stance in the wake of anger over statements by Labour leader Ed Miliband and shadow chancellor Ed Balls in support of some of the Government's austerity measures. The GMB said for this number of branches to raise concerns about the Labour leadership was "unprecedented."

Balls and Miliband made speeches and gave interviews backing the cap on public sector pay in mid-January, two weeks before the closing date for motions to be submitted by GMB branches.

The most explosive speech was made by Ed Balls to the Fabian Society, where he said it would be impossible to reverse the coalition's public spending cuts if Labour was elected in 2015.

This speech triggered a war of words between Ed Miliband and the leaders of all the major unions. Ed Miliband had been accused of "breathtaking naivety" by the Unison union for allegedly failing to stand up to the coalition.

Labour - which has been up to £10m in debt since the 2010 general election, often relies on the unions for up to 80% of its funding

The GMB's executive said in a statement on Tuesday: "The executive noted that over a quarter of motions to GMB congress from branches across the whole of the UK relate to the political stance of the union.

"The executive determined that 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 executive 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."

A union source said the number of motions on links with Labour was "unprecedented" and ranged from stepping up efforts to "reclaim" Labour, to "walking away" from it.

The GMB gives Labour more than £2m a year in affiliation fees and other funds, making it the third largest union donor to the party.

General secretary Paul Kenny wrote to his senior officials last month, saying Mr Balls' speech could have a "profound impact" on the union's relationship with Labour.

Dave Prentis, general secretary of Unison, accused Mr Miliband of "breathtaking naivete", saying there could be "unintended consequences".

Unite leader Len McCluskey warned that Mr Miliband was setting Labour on course for electoral "disaster" and undermining his own leadership.