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 tomorrow 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% 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 wasn't unanimous.
It has been estimated that 400,000 Unite members don't 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.
Mr McCluskey said he welcomed any move for trade unionists to have a more direct affiliation with Labour, saying it was part of Unite's political strategy.
"We want to get more of our members engaged with Labour at grassroots level. We see this as an opportunity and a challenge to actively talk to our members and try to persuade them to give a commitment to Labour.
"I hope we are able to persuade a number of our members to engage, but it will be an ongoing process, not just a one-off question.
"Their response will be dictated by what they perceive Labour is offering them in terms of policies, their work and their communities.
"We have a million members who pay the levy. We will have to ask them whether they are prepared to tick a box to say whether they are happy for some of their money to be given in affiliation fees to Labour."
Unite gives Labour around £3 million a year in affiliations and there has been speculation that this could be reduced by up to half.
New members will be asked immediately if they want to affiliate, but there will be a five year period for consultation with existing union members.
:: Members of the Newham Trade Unionist and Socialist Coalition will stage a protest outside the conference, urging delegates not to "silence" trade unions.
"If implemented, the Collins review will mean the destruction of the last remnants of the trade unions' organised presence within the Labour Party," said a spokesman.