McCluskey, who is head of the trade union Unite which has provided Labour with financial backing to the tune of £12m since 2010, has called for Labour not to coalesce with Nick Clegg's party should they win the general election, but not have a majority share of the votes. He still hopes the party
comes out on top in the elections, though.
"If Labour win – and I hope they win, in fact I believe they will win – if they enter into a coalition there is no doubt it would lead to a discussion about our relationship with Labour.
"To enter into a coalition in those circumstances would be very, very dangerous, given the way the Liberal Democrats have played during the coalition."
However, he has stated that there is a scenario in which he would not withdraw funding for a coalition. If Labour win the most seats in the 2015 election, but lose seats from Scotland should there be a yes vote in the independence referendum this month, they should consider working with them.
McCluskey himself has a vote in the referendum, and has
given suggestions that he is in favour of Scottish independence despite his more neutral public standpoint.
This is the latest in a line of warnings from McCluskey to Miliband. Previously he has threatened to withdraw funding should Miliband not have a
referendum on Europe, which could be a decisive factor in the general elections, with UKIP wanting to withdraw and Conservative offering a referendum. He warned that he could become someone who is "afraid of asking the British people their views", which could work against Labour in the elections.
In April, he said that he could see Labour losing the election if they offered "
variations" on the austerity-heavy policies of the Conservative party, and should they lose, he "fears for the future of the Labour Party."