Jeremy Corbyn has questioned the civil service’s neutrality after officials reportedly told a newspaper he was “too frail” to lead Labour.
The Times said it was briefed by two senior civil servants with suggestions that 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.”
He 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 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.
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 – two days after being readmitted.
Mr Williamson was suspended in February for remarks about the party’s handling of anti-Semitism.
Mr Corbyn said: “It’s an internal party matter. I cannot comment. I am not involved in the details of the case.”