Downing Street has denounced claims made by senior civil servants that Jeremy Corbyn is “too frail” to be Prime Minister.
Theresa May's official spokesman said the comments published in The Times on Saturday were “inappropriate and unacceptable”.
“Impartiality is one of the fundamental values of the civil service and underpins its ability to effectively serve the government of the day,” he said.
“It would clearly be inappropriate and unacceptable for comments of this sort to have been made or briefed to the press.”
Cabinet Secretary Sir Mark Sedwill will write to the Labour leader after the party demanded an investigation into the break of civil service protocol.
On Saturday, The Times reported it had been told by two senior civil servants that the 70-year-old may have to stand down due to health issues.
It said Mr Corbyn was being “propped up” by his advisers, and suggested he was losing his memory.
In response Mr Corbyn said the remarks were “tittle tattle”, and warned the civil service to remain neutral.
Labour MP Dawn Butler talk talkRADIO’s Matthew Wright the story was “such BS”.
“If civil servants are briefing national papers about the leader of the opposition, who could be the Prime Minister of this country, we have a serious problem,” she said.
“Civil servants have to sign a contract, they are politically neutral. It is really important that our country is run that way, so that your advice to the Prime Minister and the government doesn’t change.”