Unions are set to approve reforms to their link with Labour, but the changes will hit the number of members affiliated to the party as well as funds.
A special conference is today expected to endorse plans by leader Ed Miliband for union members to opt in to party membership rather than be automatically affiliated.
The proposal has already led the GMB to slash its affiliation funding to Labour, and Unite will discuss its funding arrangements next week.
General secretary Len McCluskey said he suspected only 10 per cent of Unite’s one million members affiliated to Labour would opt to stay in if they were asked now.
Unite’s executive has endorsed a report on the reforms drawn up by former party and union official Lord Collins, but the vote was not unanimous.
It has been estimated that 400,000 Unite members do not vote Labour, a position Mr McCluskey has said is untenable.
He told the Press Association that Unite was “honour bound” to promote a different relationship as a result of the reforms put forward by Mr Miliband.
“We have some difficult choices to make – but it doesn’t mean we could not make up any shortfall with donations.”
Unions don’t want to be seen to be threatening Labour’s finances a year before the general election even though the reforms are expected to lead to cuts in affiliation funding.