Jeremy Corbyn has said it is “very concerning” that civil servants had briefed a newspaper with allegations about his health.
The civil servants told the the Times that Corbyn, 70, could be forced to stand down because he was not up to the job “physically or mentally”.
According to the newspaper, the Labour leader was discussed at a meeting attended by White Hall officials earlier this month amid suggestions he was “losing his memory” and that he is being “propped up” by his advisers.
One shadow cabinet minister allegedly said he looked old, exhausted and was being “run ragged”.
In response to the article, which was the front page splash of the newspaper on Saturday, Corbyn said: “The idea that civil servants should be briefing a newspaper against an elected politician, against a prospective government, is something that should be very concerning to a lot of people.”
He said the civil service has to be independent, non-political and non-judgmental of the politicians they have a duty to serve.
“I would make that very clear if we were elected to government. We have a very clear framework of things we want to do in this country on housing, education, health and the environment and so many things,” he said.
“We would explain those to our colleagues in the civil service and expect them to carry out those policies. That is the way British democracy must work.”
Earlier on Saturday MPs and activists defended Corbyn against the report, pointing out that he runs more than 5km several times a week and cycles.
Labour MP Tanmanjeet Singh Dhesi also tweeted, saying: “Give us a break - where do they get these headlines from!
“Along with his parliament duties [Corbyn] is campaigning up & down the country, enough to put many of us seasoned campaigners to shame. I see & chat to him almost every week - there’s nothing frail about him.”
The shadow chancellor, John McDonnell, earlier told Sky News that Corbyn ran every day.
A party spokesperson said: “Jeremy Corbyn leads an active life, running and cycling regularly, and is in good health.
“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.”