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Jeremy Corbyn could step down if Labour’s fortunes do not improve before the 2020 General Election, Len McCluskey claims.

The head of Britain’s biggest union Unite – and a key ally of the Labour leader – believes Mr Corbyn should be given time to prove himself.

But he says: “Let’s suppose we are not having a snap election. It buys into this question of what happens if we get to 2019 and opinion polls are still awful.

“The truth is everybody would examine that situation, including Jeremy Corbyn and John McDonnell.”

He adds of the party leader and Shadow Chancellor: “These two are not egomaniacs, they are not desperate to cling on to power for power’s sake.”

Mr McCluskey warns Labour risks losing ground to UKIP if it does not sort out its position on immigration and “get its narrative right on free movement”.

(Image: Daily Mirror)

He says: “We need to expose what UKIP stand for; not just anti-foreign – anti-workers’ rights, their hidden agenda on the NHS and welfare state.

“But it will only work if ordinary people believe Labour is listening to their concerns and has solutions.”

And though he was one of Mr Corbyn’s strongest supporters in last summer’s coup bid, he says: “It’s always based on critical support. It’s my job to promote Unite’s policies, not Labour’s.”

The Liverpudlian, 66, stands for re-election this year as general secretary and rubbishes challenger Gerard Coyne’s claim he spends too much time involved in politics.

He adds: “I am disappointed it would appear as though he’s being used by the right wing of the Labour Party to try to have a proxy war about Corbyn.”

Pointing to Unite victories over Sports Direct and British Airways, Mr McCluskey says he will continue to take on “bullying bosses and bad employers” and insists: “We are fearless in defending ordinary working people.”