The leader of Unite, Labour's largest union backer, has warned that proposed changes to the party's recruitment procedures could cause a drastic cut in the union's funding.
General Secretary Len McCluskey made his views clear in a letter to its National Executive.
His comments came after Labour leader Ed Miliband outlined plans to give union members a choice about whether they want to be signed up to the party.
Mr Miliband is proposing replacing the automatic affiliation fee paid by three million union members following a major row over the party's selection process for the Falkirk seat.
Unite currently affiliates around one million members to Labour which results in fees to the party of around £3.25 million.
However, under the proposed changes which Mr Miliband wants to bring in before the 2015 General Election, the number of members affiliated to Labour could drop as low as around 75,000 after the new rule is implemented.
This would reduce fees to around £250,000.
It came after Paul Kenny, the General Secretary of the GMB, told the BBC that its affiliation fees to Labour could drop by as much as 90%.
Unite has faced accusations, which it has strongly denied, of trying to rig the selection of the party's parliamentary candidate for the constituency.
The Labour leader said changing its link with the unions would signal an end to "machine politics".
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