Senior Labour figures should back Jeremy Corbyn on Brexit or “step aside”, a powerful union leader has said.

Unite boss Len McCluskey named Emily Thornberry, the shadow foreign secretary, when he warned that shadow cabinet members needed to get behind the Labour leader.

The beginning of Labour’s annual conference in Brighton has been marred by infighting, following the shock resignation of one Mr Corbyn’s closest aides and a failed attempt to oust deputy leader Tom Watson.

Divisions over Brexit are already looming large over the proceedings, with splits between pro-EU MPs who want the Labour leader to commit to campaigning for Remain and more eurosceptic figures such as Mr McCluskey.

Speaking to Sky News’ Ridge on Sunday, Mr McCluskey said: “When we have a policy on Brexit and Jeremy Corbyn makes it clear that that is the policy, then that’s what leading members of the shadow cabinet should argue for.

“If they find that they can’t argue for it because they feel strongly, well of course they have that right – but they should step aside, step aside from the shadow cabinet, which will become the cabinet, and they can argue whatever they want.

“The policy and my appeal to them – to Emily and anybody else – is support your leader.”

Mr McCluskey said the shadow foreign secretary was a “fantastic politician” and urged her to continue to show loyalty to Mr Corbyn.

“For Labour to get through their message of unity and healing our nation, everybody needs to be singing from the same hymn sheet,” he said.

He dismissed speculation he had been behind an abortive attempt to oust Mr Watson on the eve of conference as “fake news”.

“I had nothing to do with it and I knew nothing about it,” he said.

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“Basically what it is is just frustration as the deputy leader of any organisation, the prime role is to assist the leader.

“Unfortunately, Tom gives the impression that every time he speaks it is to undermine the leader. That frustration amongst members will manifest itself, and I think that is all that happened.”

Mr McCluskey also sought to play down the resignation of Andrew Fisher, one of the Labour leader’s closest aides, who authored the 2017 election manifesto.

Mr Fisher reportedly warned that the party would not win the next general election, and walked out last Saturday.

But Mr McCluskey said he was not leaving yet, saying: “He’s made it clear that family commitments – he’s got a young family – and an incredibly high pressure job, he wants to have a better work life balance.”