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Ed Miliband today delivered a blunt warning to the trade unions that they risked a fight for survival - during a visit to the Daily Mirror.

The Labour leader said the unions had a “massive challenge” to prove they were still “relevant” in the 21st Century.

“The trade unions have a massive challenge: are they relevant to the 21st century?

"If you talk to young people they are asking ‘why should I join a trade union?’ That’s the challenge,” he said in an address to staff at Trinity Mirror, the publisher of the Daily Mirror.

His tough stance will put increased strain on relations between Labour and its biggest financial backers.

(Image: Daily Mirror)

With the TUC discussing this week the possibility of a fresh wave of strike action, Mr Miliband said the unions needed to “reflect” on the way the operated.

“Of course, there are conflicts but it’s also about co-operation and they should reflect on that,” he said.

His message came as Unite general secretary Len McCluskey launched a stinging attack on the Blairites in the shadow Cabinet and appeared to call for Liam Byrne to be removed from the work and pensions brief.

Mr McCluskey said Mr Miliband has “got to figure out what his team will be.”

“Byrne certainly doesn’t reflect the views of my members and of our union’s policy.

"I think some of the terminology that he uses is regrettable and I think it will damage Labour.

"Ed’s got to figure out what his team will be,” the union leader told the New Statesman magazine.

(Image: Daily Mirror)

Mr Miliband said it would be “madness” for Labour to break its historic link with the unions.

But he reached out over the heads of the union leadership by saying he wanted to work with the members rather than their bosses: “the nurses, shopworkers and engineers.”

Speaking later, a spokesman for Mr Miliband issued a furious put-down of Mr McCluskey.

“Len McCluskey does not speak for the Labour Party. This attempt to divide the Labour Party is reprehensible.

“It is the kind of politics that lost Labour many elections in the 1980s. It won’t work.

"It is wrong. It is disloyal to the party he claims to represent,” the spokesman said.

The Labour leader also used the meeting at the Daily Mirror’s HQ at Canary Wharf in London - the first time a serving Labour leader has addressed Trinity Mirror staff - to knock down talk of a post election Lib-Lab pact as he branded Nick Clegg a “Tory accomplice.”

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The Labour leader made clear there had been no thaw in relations between the two parties despite their joint stance on press regulation.

Asked if he could work with Mr Clegg in the event of a hung parliament, he replied by delivering a stinging attack on the Lib Dem leader’s slavish support of the Conservatives in government.

“He’s been the Deputy Prime Minister in a Government taking the country in totally the wrong direction.

“The Tories are Tories and the only way they can be Tories in Government is the Lib Dems allowing them to do so.

"They are an accomplice, not a brake,” Mr Miliband.

Mr Clegg recently suggested he could “do business” with Mr Miliband if voters again failed to return an overall winner in the 2015 general election.

But Mr Miliband made clear he would find it difficult to work with the Lib Dem leader when questioned by journalists.

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The Labour leader also praised the Sunday People for its campaign against the bedroom tax and vowed to “keep at it.”

But he refused to promise to reverse the hated measure if he wins the next election.

He also thanked the Daily Mirror for “keeping politics at the forefront of your agenda and engaging readers”.

He said the paper “embodies many Labour values and this is what unites us.”

But he was almost stumped when asked if he was “cooler” than PM David Cameron.

After some deliberation he admitted: “I don’t claim coolness. What I claim is values and integrity. Politicians who claim to be cool come a cropper.”