The brother of Labour leader Jeremy Corbyn has come to his defence, after senior civil servants claimed the politician was "too frail" to be Prime Minister.
At the weekend, 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.
But his sibling, Piers Corbyn, has insisted that the politician's health is "extremely good".
"When he and I were young, he used to do athletics. I could run faster by the way," Mr Corbyn told talkRADIO's James Whale.
"He does regular running and his fitness is high. I have seen him run and genetically he looks rather like my grandfather, who lived to be very old, to a very great age."
He continued: "The idea that Jeremy is frail in any way is untrue."
Mr Corbyn said he suspected the story had been "made up".
The Cabinet Office are now investigating where the claims came from, and Theresa May's spokesman said if identified, those responsible would face "disciplinary action".
“I should think the story was made up somewhere between a civil service and a journalist," Mr Corbyn added.
Jeremy Corbyn will meet civil service chief Sir Mark Sedwill on Wednesday afternoon to call for an independent investigation into what took place.
A spokesman for Mr Corbyn said what happened was "not only entirely unacceptable, but, I would say, unprecedented in recent times."