Tatiana Fokina made the remarks on streaming service Paramount+’s documentary Secrets of the Oligarch Wives. Ms Fokina is married to Yevgeny Chichvarkin, who amassed his wealth from a mobile phone business.
She is also the boss of a wine and spirits emporium on top of running a Michelin-starred restaurant in London, where she lives with her husband.
The couple have spoken out in the past in opposition to Putin and Russia’s war in Ukraine.
Ms Fokina says in the programme: “There are rumours that [Putin] is ill… and I think they’re likely to be true.
“And if they are true… that makes me really frightened, because a person who is really ill doesn’t really care about what happens next.”
Rumours about Putin being seriously ill started circulating before Russian troops invaded Ukraine on February 24, but the dictator’s health has come under intense scrutiny since.
Unconfirmed reports say the president has cancer or Parkinson’s disease and his health is deteriorating rapidly.
Ukrainian officials have also claimed Putin is gravely ill, but have offered no firm evidence to support this.
Russian foreign minister Sergei Lavrov recently dismissed the rumours, saying: “I don’t think that a sane person can suspect any signs of an illness or ailment in this man.”
READ MORE ABOUT COVID HOSPITALISATIONS
Putin has also been particularly cautious during the Covid pandemic, leading some to suggest his immune system is already compromised and he would be at risk if he were to catch the virus or another illness.
Russia’s leader was also photographed wrapped in a blanket during the Victory Day Parade in Moscow.
However, some have cast doubt on the rumours, including a medical expert who told Express.co.uk: “I will accept he has a round face indicative of something or maybe nothing.
“But his behaviour could be deliberately done to spoof people for something more sinister.
“It could all be put on, who knows. These are the dark arts that we are not practised in.”
Disinformation specialist Andrew Fink said Putin could be deliberately spreading fake information about his health to prevent further damage to Russia’s relations with the West.
He said some stories could be planted as part of a disinformation campaign in a bid to fracture the Western alliance with some European countries dependent on Russian exports less inclined to cut ties if they are under the impression Putin is approaching death.
Meanwhile, Prime Minister Boris Johnson told German broadcaster ZDF that if Putin were a woman, he would not have embarked on a “crazy, macho war of invasion and violence” in the way he has.
Mr Johnson said: “If you want a perfect example of toxic masculinity, it’s what he’s doing in Ukraine.”
Separately, the PM said Putin had been “proved completely wrong” in his hopes to reduce NATO dominance with Finland and Sweden moving closer to joining the military alliance.