Unite has given a further £1 million donation to Labour, bringing the total to £3.5 million since December, the union has announced.
General secretary Len McCluskey said the union was keeping to its promise that it would not stand by and see a one-sided election fight against a Conservative party "made fat on the gifts from the shady and the super wealthy."
He said the money from Unite members was "clean, democratically-sanctioned and honestly accounted for".
Mr McCluskey said that in contrast, the Tories get millions of pounds from "shadowy dinner clubs and vulture fund financiers".
The move comes just days after Unite criticised some Labour officials for trying to fuel a row between party leader Ed Miliband and union general secretary Mr McCluskey.
"David Cameron has shown that he is utterly clueless about the troubles that his government have caused for the good people of this country. People working hard but only just getting by, or forced to the foodbank when the wages won't stretch. People desperate to see a doctor but waiting weeks for an appointment. Kids denied a chance to use their talents stuck in dead-end work. It is terrifying to think of what five years of Tory government could do to their hopes.
"This is a fight for the poor and vulnerable. A fight for everyone squeezed by the crisis and the cuts, and for everyone who believes that Britain has gone badly wrong, and who wants to live in a fairer country.
"Support for Labour is not just us doing our duty to democracy and ensuring the political playing field is not tilted too far in favour of the rich and their Party. We also know that that if the Tories buy a new lease on power, life for the people we serve will become very much harder," said the Unite leader.
Writing for the Daily Mirror online, he said: "Unite said that it would not stand by and see a one-sided election fight against a Tory party made fat on the gifts from the shady and the super-wealthy, and we are keeping to that promise."
There has been tension in the past week between Labour and Unite after one of the union's activists, Karie Murphy, failed to be selected for a party shortlist for the marginal seat of Halifax.
Ms Murphy was at the centre of a vote-rigging row in Falkirk last year although she, and Unite, were cleared of any wrong-doing.