The head of Britain's biggest union has warned it could cut ties with Labour unless the next leader acts as the "voice of ordinary people".

Unite boss Len McCluskey said it was essential that the "correct" choice was made in the battle to succeed Ed Mililband.

If the next to take the helm failed to stem "disillusionment" among members of the union then it might rethink its affiliation.

The comments - which raise the prospect of Labour losing millions of pounds in funding - came as tensions over the party's future direction burst into the open in the wake of the devastating election defeat.

Jim Murphy announced yesterday he was quitting as leader in Scotland - and delivered a parting shot at Mr McCluskey in which he branded him the "kiss of death" for the party.

Denouncing a campaign to oust him from his post, Mr Murphy said: "Whether it's in Scotland or the contest to come across the UK, we cannot have our leaders selected or deselected by the grudges and grievances of one prominent man.

"The leader of the Scottish Labour party does not serve at the grace of Len McCluskey and the next leader of the UK Labour party should not be picked by Len McCluskey."

But speaking on BBC Radio 5 Live show Pienaar's Politics, Mr McCluskey said he was being portrayed as a "bogeyman".

"Jim is hurting and I can understand that. But he's playing the same trick that rightwing media has played for a number of years, looking for a bogeyman as an excuse. I wasn't the one who lost Scotland to the SNP."

The Unite chief - who pointed out that more of his members in Scotland voted SNP than Labour - said it was "essential that the correct leader emerges, and that there's a genuine debate about the direction we are going in".

"It is the challenge of the Labour party to demonstrate that they are the voice of ordinary working people, that they are the voice of organised labour," he said.

"If they do that in a way that enthuses us then I don't believe that the mountain that is ahead of us in unclimbable.

"It's up to them. If they inject more disillusionmnent in the party then the pressure will grow from our members to rethink."

Mr McCluskey refused to confirm that his favoured candidate was frontrunner Andy Burnham, saying there would be an intensive campaign and hustings.

Speaking on the BBC's Andrew Marr Show, Mr Burnham insisted he was "proud" of the party's link with the unions - but stressed Mr McCluskey would only have one vote in the leadership contest.

"This won't be union bosses deciding, this will be school dinner ladies, this will be healthcare assistants," he said.

"I am the unifying candidate. I am bringing all parts of the Labour movement together."

Mr Burnham also indicated Mr Murphy had been right to resign.

"I've got huge respect for Jim, he's given his all to the Labour Party, but in Scotland we do now need a clean break, we need a process of listening and learning and rebuilding and I believe in this contest I'm the person best placed to do that," he said.