Jeremy Corbyn must start showing leadership and stop making "inappropriate" comments, his close ally and Unite chief Len McCluskey has said.

In a surprising intervention from one of Mr Corbyn's biggest union backers, Mr McCluskey said the Labour leader was used to being a backbench MP but must now realise he can no longer "say what he likes".

He pointed to Mr Corbyn's initial comments opposing 'shoot-to-kill' following the terror attacks in Paris as an example of him failing to react appropriately in his new role as leader.

He added that Mr Corbyn only had a chance of becoming Prime Minister if he put forward a "credible economic alternative" that had widespread support amongst voters.

His comments, first reported by the York Press, follows criticism from Unison general secretary Dave Prentis, who also backed Mr Corbyn for the leadership but told The Independent on Sunday that it was time the leadership "got its act together" before it lost any chance of returning to power in 2020.

Speaking at an event at the University of York, Mr McCluskey said: "Jeremy Corbyn has to come to terms with it [his leadership].

"He has been a very principled MP and been able to say what he likes, but now he's a leader and in leadership he can't necessarily say the first thing that comes into his head. He has to take some balance."

As Labour faces the very real danger of losing a safe seat to Ukip in the Oldham West and Royton by-election next week, Mr McCluskey urged Mr Corbyn to tackle the threat of Nigel Farage's party head-on by taking on "greedy bosses" who exploit cheap Eastern European migrants to undercut British workers.

He added: "The only way for Jeremy Corbyn to become Prime Minister is if he puts forward a credible economic alternative that the British people can sign up to."

Mr McCluskey later issued a statement insisting he was fully behind Mr Corbyn’s leadership.

“Jeremy has my full support as he develops his alternative programme to that of this disastrous government," he said. "He has opened up debate and democracy across the Labour party and that can only be a positive move for the future.