The former British spy who wrote a dossier on Donald Trump and alleged Russian interference in the 2016 US election campaign says sources have told him Vladimir Putin is "seriously ill".
Christopher Steele, who ran the Russia desk at MI6 in London between 2006 and 2009 and worked there in the 1990s, said Putin's illness was "an element" of what is happening in Ukraine.
He told Sky News: "Certainly, from what we're hearing from sources in Russia and elsewhere, is that Putin is, in fact, quite seriously ill.
"It's not clear exactly what this illness is - whether it's incurable or terminal, or whatever. But certainly, I think it's part of the equation."
His comments come after Ukrainian Major General Kyrylo Budanov, in an exclusive interview with Sky News, also said the Russian leader is seriously ill with cancer and that a coup to remove him is under way in Russia.
Speculation around Vladimir Putin's health has circulated for several years, but has only intensified since he ordered the invasion of Ukraine.
New Lines magazine reported it has obtained an audio recording of an oligarch close to the Kremlin who describes the Russian president as "very ill with blood cancer", although the type of blood cancer was not specified.
It says a "top-secret memo" was sent out by the headquarters of the FSB, Russia's domestic security agency, to all its regional directors instructing regional chiefs not to trust rumours about the president's terminal condition.
Mr Steele added: "When you see that happening, you think it's probably true. So, I think there is an element of his illness involved in this and his legacy."
The rumours intensified on Wednesday when Vladimir Putin's address during Russia's Victory Day commemorations was closely scrutinised, and his movements and physical appearance were studied by observers.
Body language experts pointed out that the president's face was "puffy" and his walk "unsteady", which some have suggested could signify some form of medication for an illness.
But Mr Steele said, even if he was ill, it may still be difficult for anyone else to have any impact on Mr Putin's approach, even though he agreed with the kind of sanctions now being employed, like those on his ex-wife and cousins.
The British government said its latest asset freezes and travel bans targeted the "shady network" of friends and allies who "owe Putin their wealth and power, and in turn support Putin and his war machine."
Among those hit by sanctions are Mr Putin's ex-wife Lyudmila Ocheretnaya; former Olympic gymnast Alina Kabaeva, who is "alleged to have a close personal relationship with Putin," according to the government; and several businessmen who are cousins of the Russian president.
Mr Steele added: "In general there are very few people who are prepared to stand up to or to argue with President Putin... I do think though that there are dissident voices, discordant voices, people telling him this is a disastrous war, that, particularly on the economy, will not play out well for Russia, and we can only hope that that will lead to some kind of change of policy, or even change in regime in due course, but it's certainly not a given."
German Chancellor Olaf Scholz, who spoke at length by phone with the Russian leader on Friday, said in an interview published on Saturday that he had not detected any change in Mr Putin's stance recently.
Western military analysts say Mr Putin and his generals failed to anticipate such fierce Ukrainian resistance when they launched the invasion in February.
As well as losing large amounts of personnel and military equipment, Russia is having to deal with heavy economic sanctions.
The G7 pledged in a statement on Saturday to "further increase economic and political pressure on Russia" and to supply more weapons to Ukraine.