Unite boss Len McCluskey has threatened to withdraw support from the Labour Party unless it elects the “correct leader”.
Unite donated over £1.3m to the party, making the union the second-highest donor of the fourth quarter of 2014.
McCluskey told BBC 5Live: “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. If they do that in a way that enthuses us I don’t believe that the mountain that is ahead of us is unclimbable.
“If they don’t, they inject more disillusionment, the pressure will grow from our members to rethink.
“It’s essential that the correct leader emerges, and that there’s a genuine debate about the direction we are going in. Let’s have the debate.”
“The new leader’s challenge is to make sure the voice of Labour is the voice of ordinary people.”
McCluskey also went on to say that the left needed to “challenge” UKIP. “That means in my opinion we need to seriously debate the free movement of labour,” he said.
McCluskey also slammed claims by outgoing Scottish Labour leader Jim Murphy that he was the “kiss of death” for the party.
He likened Murphy’s election to a “death sentence” for Labour.
“He has been at the epicentre of an ideology that alienated the Scottish working class for years and years and years,” he said.