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.
Why are you making commenting on The Herald only available to subscribers?
It should have been a safe space for informed debate, somewhere for readers to discuss issues around the biggest stories of the day, but all too often the below the line comments on most websites have become bogged down by off-topic discussions and abuse.
heraldscotland.com is tackling this problem by allowing only subscribers to comment.
We are doing this to improve the experience for our loyal readers and we believe it will reduce the ability of trolls and troublemakers, who occasionally find their way onto our site, to abuse our journalists and readers. We also hope it will help the comments section fulfil its promise as a part of Scotland's conversation with itself.
We are lucky at The Herald. We are read by an informed, educated readership who can add their knowledge and insights to our stories.
That is invaluable.
We are making the subscriber-only change to support our valued readers, who tell us they don't want the site cluttered up with irrelevant comments, untruths and abuse.
In the past, the journalist’s job was to collect and distribute information to the audience. Technology means that readers can shape a discussion. We look forward to hearing from you on heraldscotland.com
Comments & Moderation
Readers’ comments: You are personally liable for the content of any comments you upload to this website, so please act responsibly. We do not pre-moderate or monitor readers’ comments appearing on our websites, but we do post-moderate in response to complaints we receive or otherwise when a potential problem comes to our attention. You can make a complaint by using the ‘report this post’ link . We may then apply our discretion under the user terms to amend or delete comments.
Post moderation is undertaken full-time 9am-6pm on weekdays, and on a part-time basis outwith those hours.
Read the rules hereComments are closed on this article