Unite boss Len McCluskey has called reports about Jeremy Corbyn’s failing health “fake news”.
Speaking on BBC One’s Andrew Marr Show, the union chief said the Labour leader was “fit as a fiddle” and journalists at The Times “should be ashamed of themselves”.
The newspaper reported on Saturday that senior party figures were concerned Corbyn had suffered a mini-stroke and that civil servants did not regard the opposition leader as “mentally or physically” fit to be prime minister.
“They ought to be ashamed of themselves,” McCluskey said of the journalists behind the Times report. “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...
“I don’t believe them at all - it’s the Murdoch press, of course they are going to attack and try to undermine Jeremy.”
Corbyn himself has called for an inquiry into the briefings to the paper by civil servants.
He said the briefing of a newspaper by senior officials against a politician “should be very concerning” to people.
“The civil service has to be independent,” he said, adding: “It has to be non-political and has to be non-judgemental of the politicians they have a duty to serve.
“I would make that very clear if we were elected to government.
“There must be an investigation into which senior civil servants are spreading fictitious information to the press and in the process compromising the integrity of the civil service.”
McCluskey also launched an attack on left-wing commentator Paul Mason, who he claims has demanded all of the officials around Corbyn be sacked, saying he “has lost his marbles”.
It comes amid growing disquiet in the Labour Party ranks over the party’s Brexit position, with the leadership resisting calls to back a second referendum and campaign for remain.
There should be no “panic” to switch Labour’s position on Brexit, McCluskey added, as he blamed a “well-funded Remain lobby” for creating a “toxic situation”.
He said: “There seems to be a panic to rush in order to establish a different position from one the Labour Party has had for a couple of years now, which is respecting the result of the 2016 referendum and trying to negotiate a deal which would unite the nation.
“Unfortunately we have had a prime minister who has made huge mistakes and a government which has been incapable of delivering Brexit.
“And we have had a well-funded Remain lobby that has turned the nation into a toxic situation.”
Shadow chancellor John McDonnell, meanwhile, told Sky’s Sophy Ridge that shifting Labour’s stance to full support of Remain and a second referendum is taking longer than expected.
McDonnell said the discussion was about the “attitude” Labour should take and Jeremy Corbyn had given trade unions more time to decide.
He said: ”(Jeremy) had a meeting on the Monday before the last shadow cabinet and I thought the next day we were going to move forward.
“The unions asked for a bit more time and I don’t mind that - that’s the type of leader Jeremy is, he is a consensus builder.”