James Hewitt is fighting for his life after suffering a heart attack and a stroke, according to reports.
The 59-year-old and former lover of Princess Diana is said to be recovering after needing emergency surgery for which doctors gave him a "slim chance" of survival.
He is believed to undergoing treatment at the Royal Devon and Exeter Hospital, where he is expected to remain for several weeks.
He originally sought medical help after falling ill two weeks ago, according to concerned friends.
According to the Mail Online, a relative told a Sunday paper: "The family were concerned. He had a pre-existing illness, then got seriously ill. Suddenly it got very critical and he was rushed to hospital.
"It was pretty serious but hopefully we are all OK now. He is recovering and has been moved to Exeter."
The former army captain and the late Princess Diana infamously had a five-year affair while she was married to Prince Charles.
Originally meeting at a part in London, their dalliance finally came to an end after he was sent to Iraq in the 1991 Gulf War and split after their affair was made public.
Addressing claims that he fathered Prince Harry as a result of the romance, the war veteran remained staunch in a candid interview with Australian Channel Seven's Sunday Night.
Asked by Australian host Melissa Doyle: "Are you Harry's father?" he responded emphatically. "No, I'm not."