Jeremy Corbyn has called for a government investigation after a national newspaper reported claims about his health, allegedly based on information from civil servants.
Mr Corbyn said it is "very concerning" that civil servants had briefed the Times with details of his health as he rubbished the claims.
Anonymous civil servants reportedly told the newspaper that the 70-year-old could be forced to stand down because he was not up to the job “physically or mentally”.
In a statement issued to the press, Mr Corbyn said: "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.”
Speaking earlier on Saturday afternoon, he 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 continued: "The civil service has to be independent. It has to be non-political and has to be 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.
"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."
Labour allies have rushed to support their leader, stating he is physically active and in good shape. He has previously written articles about he love of cycling to work in Westminster and around his constituency in north London.
Earlier this year, Labour said Mr Corbyn had received treatment at a London hospital for muscle weakness affecting one of his eyes.
In a statement, the Labour Party said: "These Times stories are a series of false, fabricated and absurd allegations hiding behind anonymous sources with a transparently political agenda.
"For any senior civil servant to falsely claim the leader of the opposition is ill, frail or forgetful, is a disgrace and a clear political intervention. In a democracy, the people decide who is prime minister.
"The claim that Jeremy Corbyn is ill, frail or forgetful is entirely false, and clearly part of a deliberate misinformation campaign. Jeremy is in good health, runs and cycles regularly, and has an excellent memory."