Ed Miliband has slapped down the leader of Labour's biggest trade union donor, accusing him of disloyalty and a "reprehensible" attempt to divide the party.

The furious Labour leader turned his fire on Len McCluskey after the Unite general secretary launched an outspoken attack on a number of Shadow Cabinet ministers, saying if Mr Miliband let himself be swayed by his Blairite colleagues, then he would be "cast into the dustbin of history".

In response, a spokesman for Mr Miliband – who was elected to the party's top job on the back of trade union votes – said: "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."

In an interview with the New Statesman magazine, Mr McCluskey urged Mr Miliband to be brave and do something radical, noting: "If he gets seduced by the Jim Murphys and Douglas Alexanders, then the truth is he'll be defeated and he'll be cast into the dustbin of history."

He added: "If he is daft enough to get sucked into the old Blairite 'neoliberalism wasn't too bad and we just need to tinker with it a little bit', then not only will he fail but I fear for the future of the Labour Party."

The union chief appeared to suggest he wanted the removal from the Shadow Cabinet of certain Blairites, who also include Liam Byrne, the Shadow Work and Pensions Secretary, saying: "Ed's got to figure out what his team will be."

Mr McCluskey urged the Opposition leader to go into the election with "a team that he's confident in".

He said: "Ed Miliband must spend most of his waking hours grappling with what lies before him. My fear is that if he is simply offering the British electorate an austerity-lite programme, then it won't capture their imagination."

His comments seem to be a direct response to those made earlier this month by Tony Blair and other New Labour figures, who warned Mr Miliband not to take Labour to the Left. The former Prime Minister said Labour was in danger of being seen as a 1980s party of protest, urging Mr Miliband not to allow it to slip back into its "comfort zone".

Last night, Mr Murphy, the Shadow Defence Secretary, hit back at the Unite leader and said: "It's disappointing in advance of important local elections that Len McCluskey turns his fire on Labour."

Unite has given £7 million to Labour since the 2010 general election.