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Unite leader Len McCluskey has insisted “there is no panic” over Labour’s Brexit policy and the party should trust leader Jeremy Corbyn.
He told the BBC it should not rush to back remaining in the EU and should wait for the party’s consultation.
But shadow chancellor John McDonnell told Sky he was “a little” frustrated over the delay in deciding whether to oppose Brexit at the next election.
Mr McCluskey dismissed reports about Mr Corbyn’s health as “fake news”.
Speaking on the BBC’s Andrew Marr Show, he said some people were in a “rush” to change Labour’s position of “respecting the 2016 referendum and trying to negotiate a deal which would unite the nation”.
He blamed “huge mistakes” by Prime Minister Theresa May, a government “incapable” of delivering Brexit and a “well-funded Remain lobby” for turning the Brexit debate “toxic”.
But he denied that it was time for Labour to support remaining in the European Union.
“There is no panic, there is no panic to do anything. Let Jeremy Corbyn consult,” he said.
“My message to Labour MPs and members is he’s done OK so far, let’s trust him to consult and see what emerges.”
Mr McCluskey said he wanted to see a general election with a new Labour government negotiating a fresh Brexit deal, which would then be put to a referendum.
Asked what he would choose in a referendum between a no-deal Brexit or remain, he said he would back remaining in the EU.
“We are absolutely fundamentally opposed to no deal,” he said.
‘Lies’ and ‘distortions’
Mr McDonnell told Sky News’ Ridge on Sunday that he had been arguing “we need to move now” in changing the party position to explicitly campaign for staying in the EU in a future referendum.
He said he was “a little bit” frustrated, but added: “Jeremy is the type of leader I completely support and think we need now, he’s a consensus builder.”
Asked about a report in the Times that two civil servants had said Mr Corbyn was “too frail” to become prime minister, Mr McCluskey said it was “disgraceful” and untrue.
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The union leader said: “It was fake news, it was lies, it was distortions. Jeremy Corbyn is as fit as a fiddle, he’s one of the strongest individuals I’ve ever met, people 20 years younger can’t keep up. There’s nothing wrong with Jeremy.”
He said he had faith in the civil service’s impartiality, but if there was “any element of truth” that civil servants had made the comments, there should be an investigation.
Jon Trickett, Labour’s shadow minister for the Cabinet Office, has written to the head of the civil service, Mark Sedwill, calling for an independent investigation into what he called the apparent breach of civil service neutrality.
He said: “It is hard to avoid the conclusion that this has been a totally unwarranted and indeed unconstitutional political intervention with disturbing implications for our democratic system.”
