Media playback is unsupported on your device Media caption Len McCluskey: "Let there be no doubt - Unite stands fully behind Labour and Ed Miliband"

The UK's biggest trade union says it will fund Labour's election campaign, saying it will not let the party fight with "one hand tied behind its back".

Unite general secretary Len McCluskey said members faced "the fight" of their lives to oust the "ruinous coalition" of the Conservatives and Lib Dems.

The union has had a series of rows with Labour, but says the party needs money to end "mindless austerity".

The Tories called it the "same old Labour", dominated by unions.

Unite was one of a number of trade unions that were unhappy with reforms to the historic link with the Labour Party.

So let there be no doubt - Unite stands fully behind Labour and Ed Miliband in the increasingly radical agenda he has outlined Len McCluskey, Unite

In April this year, Mr McCluskey warned Labour leader Ed Miliband - who introduced the Labour-trade union reforms - that Unite could break its links with Labour if the party lost the next election.

He said he could see the union voting to disaffiliate from a defeated Labour if it ceased to be the voice of working people.

However, in his speech to union members in Liverpool on Monday he said Labour needed to have enough funding to mount a strong campaign against the Conservative in May 2015's general election - arguing it offered the best hope of draining the "swamp of ill treatment", the chance to improve jobs and reverse privatisation of health services.

"The most important challenge Unite will face over the next 11 months is winning next year's general election," he told members.

Now is not the time "to have heated arguments" within the Labour about policy or the party's future, he said.

Image copyright Reuters Image caption Unite's backing for Ed Miliband comes despite the union cutting affiliation fees to the party

"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.

"So let there be no doubt - Unite stands fully behind Labour and Ed Miliband in the increasingly radical agenda he has outlined. It is a people's agenda and this union will be proud to fight alongside Labour to secure it."

Immigration fears

But Mr McCluskey said in return Mr Miliband had to understand that he would only win the trust of the electorate if he rejected "the siren voices of austerity lite" to be on the side of ordinary working men and women.

"We will not let Labour fight for this programme with one hand tied behind its back," he said. "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 lop-sided because democracy demands a fair fight."

Mr McCluskey also condemned the Conservatives and UKIP for "exploiting people's fears" over immigration.

"If others are afraid to enter the battlefield with UKIP - we are not. Unite stands ready to take them on for what they are - a right-wing menace to our communities and our society."

He said a Tory win at the next election would threaten trade union freedom.

Strike action

"It's no exaggeration to say that the future of our society, and the future of the labour movement, will be in the balance over the next 12 months," he said.

"Let there be no doubt - we're now facing the fight of our lives."

Unite is Labour's biggest donor but announced plans to cut the amount it pays the party in affiliation fees by half - to £1.5m - following reforms to the Labour-union funding link.

Mr Miliband's proposals for a "one member, one vote" system for leadership elections and an end to the automatic affiliation of union members were approved at a conference of party members earlier this year.

The union has also been involved in rows with the party over allegations of the rigging of a Labour candidate selection by Unite in Falkirk, central Scotland, as well as Labour's support for public sector pay restraint.

Unite is also set to reveal whether its 70,000 members in local government in Wales, England and Northern Ireland have voted to take industrial action over pay -

Members of several major unions - including council workers, school support staff and teachers - have already voted to strike on 10 July.

More than a million public sector workers could walk out as part of a protest over government policy on cutting public sector costs and jobs.

Conservative Party Chairman Grant Shapps said: "Ed Miliband promised to wean himself off these union bosses, but he has been too weak to deliver.

"Frankly, it's the same old Labour - dominated by unions who want more spending, more borrowing and more taxes: all things that would crush British business and cost jobs."