LABOUR has called for an independent investigation into claims by senior civil servants that Jeremy Corbyn is too physically and mentally frail to be prime minister.

The warnings were leaked to The Times earlier this week, provoking fury from his MPs.

Yesterday, one newspaper even claimed Corbyn had suffered a minor stroke about three months ago.

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In a letter to Mark Sedwill, the head of the civil service, Labour’s shadow minister for the Cabinet Office Jon Trickett said the “unconstitutional political intervention” from the supposedly neutral mandarins had “disturbing implications for our democratic system”.

The Times report quoted two anonymous senior civil servants who said they were concerned about Corbyn being put into Downing Street.

They claimed he was being “propped up” by his advisers.

“There is a real worry that the Labour leader isn’t up to the job physically or mentally but is being propped up by those around him. There’s growing concern that he’s too frail and is losing his memory. He’s not in charge of his own party,” one claimed.

Another asked: “When does someone say [he] is too ill to carry on as leader of the Labour Party let alone prime minister? There must be senior people in the party who know that he is not functioning on all cylinders.”

In his letter to Sedwill, Trickett wrote: “Discussion of these matters, based on false assumptions, should not be taking place. Worse, it is without precedent in my experience that any high-level discussion about senior politicians, let alone the leader of the opposition, should be shared with a newspaper.

“I must now formally say to you that 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.

“There clearly needs to be an investigation into what appears to have been a breach of civil service neutrality, independent of the Cabinet Office, in order to avoid any real or apparent conflicts of interest.”

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Unite union leader Len McCluskey, a close ally of Corbyn, told BBC One’s Andrew Marr that the journalists responsible for writing the stories “ought to be ashamed of themselves,” he said. “It was fake news, it was lies, it was distortion.

“Jeremy Corbyn is fit as a fiddle, he is one of the strongest people I have ever met – people 20 years younger struggle to keep up with him.”