Labour's biggest affiliate is to cut its funding by £1.5 million in the wake of changes to the party's links with trade unions.
Unite said it will affiliate 500,000 members to the party in 2014 and will review the number annually.
The union had affiliated one million members, worth £3 million a year to Labour, so the move will effectively cut funding by around £1.5 million.
The GMB union decided last year to cut its affiliation funding by £1 million because of reforms put forward by Labour leader Ed Miliband.
A special conference agreed at the weekend to adopt the changes, under which union members will have to opt in to party membership rather than be automatically affiliated.
The reforms were put forward by Mr Miliband following controversy last year over Unite's involvement in the selection of a Labour candidate in Falkirk.
A statement by Unite said the decision of the special conference sets the collective relationship of Unite and other affiliated unions with the party on a "new course."
Unite's executive also authorised general secretary Len McCluskey to respond to any requests for additional financial assistance from Labour, with a general election just over a year away.
The statement said: "The union will rapidly prepare a plan to ensure that we maximise the number of our political levy-paying members who express support for our continuing collective affiliation, and who take advantage of the possibility of becoming associated members of the party.
"Our representative on the implementation committee which will oversee the introduction of the agreed reforms will, among other things, work to ensure that the interests of our members are protected in the forthcoming selection process for Labour candidate for London mayor and in any leadership election that may occur before 2020.
"When the leader of the party announced his intention to seek changes in the Labour-union relationship in summer 2013, he made it clear that he did not think it appropriate that the party continue to accept affiliation fees from those who had not actively assented to such payments. Unite accepted that principle at the time."
The new opt-in arrangements will apply immediately for new union members, and within five years for existing members.
The statement continued: "By the conclusion of the transitional period in 2020, it will be clear how many members of Unite and other unions actively support their political levy being used to affiliate to the Labour party. Unite will work to make that number as large as possible.
"It is inevitable, however, that the final total will be considerably less than the present one million members affiliated. Opinion polling evidence suggests that, while Labour is by some way the most popular choice for Unite members at the ballot box, no more than half the membership in Britain vote Labour at present (many of the others not voting at all).
"The Executive Council therefore agrees that Unite's affiliation will need to be reduced over the five-year period to 2020 to reflect this reality. It will therefore affiliate 500,000 members to the party for 2014, and will review this number annually.
"The Executive Council is, however, also aware that we are now just a year from a general election, in which it is vital that the British people are offered a clear political alternative to the ruinous economic and social policies of the coalition government. It is not in the interests of democracy itself for Labour - the only party which can offer such an alternative - to contest the election without the resources required to make the contest a "fair fight" against the parties of global capital and the super-rich.
"Bearing in mind the tight timescales in which decisions may need to be made over this next period, it therefore authorises the general secretary to respond to any requests for additional financial assistance, beyond the affiliation fee, which may be made by the Labour party, after first consulting the Executive Council or the finance and general purposes committee."
There have been estimates that Labour could lose many millions of pounds in affiliation funds from unions because of the new arrangements.
But Mr Miliband made it clear at the conference last weekend that he wanted to encourage greater participation in the party and attract new members.
His reforms received massive backing from constituency parties, MPs and unions, although some delegates warned they could weaken the historic link with unions.
A Labour Party spokesman said: "We have always acknowledged that the ambitious reforms to change the Labour Party and the way politics is done in this country would have financial consequences. But we want Labour to become a party for millions, not just for millionaires like the Tories under David Cameron.
"Our members and small donors already give us more money than the trade unions, and the introduction of affiliated supporters and the growth of registered supporters gives us new opportunities to change our funding base in favour of many small donors rather than a few big ones."