Jeremy Corbyn has called for an inquiry after a newspaper reported two civil servants had said he was "too frail" to lead Labour or become PM.
The Times said it was briefed by two senior civil servants with suggestions the Labour leader may have to stand down over supposed health issues.
Mr Corbyn has called it "a farrago of nonsense" and "tittle tattle".
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."
Mr Corbyn, who was at an event to mark Armed Forces Day, added: "I am a very fit, healthy active person. I love what I do and I love my community and love being outdoors."
The article in the Times - published on Saturday - reported an official saying there was a "real worry" the 70-year-old is not up to the job "physically or mentally", is "losing his memory" and is being "propped up by those around him".
The unnamed official is quoted as saying: "There's growing concern that he's too frail and is losing his memory. He's not in charge of his own party."
The BBC has not verified the quotes made in the newspaper.
Jeremy Corbyn has dismissed the Times front page questioning his fitness for office as "tittle tattle".
So why draw more attention to it by responding so robustly and questioning the role of civil servants?
There is a genuine concern by those around the Labour leader that there is an attempt to undermine him by senior Whitehall figures.
One insider told me the Times story was regarded by the party leadership as "sinister". That's because it went beyond the usual political attacks and was deeply personal. And that not to respond to the accusations would only lead to yet more negative briefing.
So there is desire to "nip this in the bud" - and to convince the current government to remind civil servants of their duty of impartiality.
And if they don't, then Jeremy Corbyn's base will rally to him against threats by those seen as "the establishment".
Earlier, Labour denied the claims, saying suggestions Mr Corbyn does not make his own decisions are "laughable" and "demonstrably false".
A spokesman said Mr Corbyn runs and cycles regularly, and added: "Reports to the contrary are scurrilous and a transparent attempt to undermine Labour's efforts to redistribute wealth and power from the few to the many."
Mr Corbyn was also asked about MP Chris Williamson, who has been suspended from the party for a second time over the anti-Semitism row - two days after being readmitted.
Mr Williamson had been suspended in February for remarks about the party's handling of anti-Semitism.
He was allowed back in on Wednesday after an investigation by the party's National Executive Committee. A Labour source has said the decision is still pending.
Mr Corbyn said: "It's an internal party matter. I cannot comment. I am not involved in the details of the case."