The leader of Britain's biggest union has pledged his full support to Labour, saying the party will not be allowed to fight the general election with "one hand tied behind its back" through lack of funds.

Len McCluskey, general secretary of Unite, said the union will make sure next May's election was not "financially lopsided".

He told Unite's national conference in Liverpool that the union stood "fully behind" Labour and leader Ed Miliband and his "increasingly radical" agenda.

Unite and other unions were unhappy with Mr Miliband's controversial reforms of the historic link between the two wings of the movement, which were approved earlier this year.

Unite cut its funding by £1.5 million a year, but Mr McCluskey sent a strong message that the union will give financial support to Labour in the run up to polling day.

"The most important challenge Unite will face over the next 11 months is winning next year's general election. There is a time to have heated arguments within the Labour party about policy. There is even a time to discuss the future of the party itself - but that time is not now, " he said.

"We have a clear and vital choice before us - it's whether we can evict the present ruinous Conservative coalition from office and get a Labour prime minister into Downing Street. There is no third option.

"We will not let Labour fight with one hand tied behind its back. We will be up against the party of the rich, bought and paid for by the rich, with its coffers overflowing with cash from hedge funds, the City and those doing very nicely out of health privatisation. Unite will do its bit to make sure that the next election is not financially lopsided because democracy demands a fair fight."

Mr McCluskey urged the Labour leader to ignore "trivial nonsense" such as comment on how he eats a bacon butty and concentrate on tackling issues such as zero hours contracts, privatisation of the NHS and inequality.

He also condemned the Tories and Ukip over immigration, saying they were "exploiting" people's fears that they were being left out of the economy.

"We must challenge Ukip which uses anti-immigration rhetoric to mask the fact that it is anti-union, pro-big business, anti-NHS and more Thatcherite than Thatcher herself. Ukip are dangerous not because they will make any significant inroads into the corridors of power, but because they make racism and prejudice seem respectable."

Mr McCluskey told the 600 delegates that for the past two years Unite had been under the most sustained attack any union had endured for a generation.

He said it was a "back-handed" compliment to the work of Unite but added that the attacks formed part of a "dangerous agenda", vilifying trade unions, and giving the message that Britain was "open for exploitation".

In a brief reference to the controversy surrounding Unite's involvement in the selection of a Labour candidate in Falkirk last year, Mr McCluskey said: "I don't want to scratch that scab because Unite and the Labour party have moved on, but let me repeat, your union did nothing wrong in Falkirk.

"It is not a crime for ordinary men and women to join the Labour party and seek to promote their views."

The Unite leader said that if the Conservatives win the next election, it will move to put many of the normal functions of trade unions outside the law.

He said: "The Conservative party should not assume that we will put respect for unjust laws ahead of our duty to fight exploitation and ruthless employers. We will fight for our members within the law where possible, but let the Tories be in no doubt that if they push us outside the law, they will be responsible for the consequences, not us."