Yevgeny Prigozhin, the head of Wagner Group, was on the list of passengers on a plane that crashed near Tver, 180 kilometres north-west of Moscow, two months after the mercenary group mounted a mutiny against the Russian military.
All 10 people on board died, according to authorities, who said eight bodies had been found at the crash site. They included Dmitry Utkin, Mr Prigozhin's right-hand man who helped found the paramilitary organisation. Forensics investigators erected a tent at the site and were seen removing the bodies in black bags on Thursday.
A Telegram channel linked to Wagner, called Grey Zone, declared the group's leader dead soon after the incident. The post hailed him as a hero and a patriot who, it said, had died at the hands of unidentified people it called "traitors to Russia".
The Kremlin and Russian Defence Ministry have yet to comment.
But Russian President Vladimir Putin's former spokesman, Sergei Markov, on Thursday blamed Ukraine for the plane crash.
Speaking to BBC Radio 4's Today programme, Mr Markov said: "For me it is absolutely clear that Prigozhin has been killed by the Ukrainian intelligence service as some kind of gift to [Ukrainian President Volodymyr] Zelenskyy for the independence day of Ukraine, which they are celebrating today."
At about 5pm local time, Russian authorities announced a “private Embraer Legacy aircraft travelling from Moscow to St Petersburg crashed near the village of Kuzhenkino in the Tver region”.
Wagner chief Prigozhin presumed dead in plane crash - in pictures
Russian authorities are investigating the circumstances surrounding the crash, the press service of the Tver regional government said on its website.
Meanwhile, Mr Putin was giving a speech on the 80th anniversary of the Battle for Kursk in the Second World War.
He did not mention the crash but hailed "all our soldiers who are fighting bravely and resolutely" in Ukraine.
In Washington, US President Joe Biden said of the crash: “You may recall, when I was asked about this by you, I said I’d be careful what I rode in. I don’t know for a fact what happened but I’m not surprised.”
Asked if Mr Putin was behind it, he said: “There’s not much that happens in Russia that Putin's not behind.
“But I don’t know enough to know the answer.”
US national security spokeswoman Adrienne Watson said on X, formerly known as Twitter: “We have seen the reports. If confirmed, no one should be surprised.”
The UK government said it was monitoring the situation closely.
British MP Alicia Kearns, chairwoman of the foreign affairs committee, said on X: “The speed at which the Russian Govt has confirmed Yevgeny Prigozhin was on a plane that crashed on a flight from Moscow to St Petersburg should tell us everything we need to know.”
Polish Foreign Minister Zbigniew Rau said on state news channel TVP Info: “We would have great trouble naming anyone who would intuitively think this was a coincidence.”
Wagner forces aim to topple Moscow's military leadership – in pictures
At the time, Mr Putin denounced the rebellion as “treason”, only to quickly agree to a deal that led to the mercenary boss being exiled to neighbouring Belarus.
Wagner troops were brought in by Moscow to provide extra firepower in Ukraine after Russian plans for a swift capture of Kyiv went awry.
Mr Prigozhin, a former Kremlin confidant and restaurant tycoon, was once nicknamed “Putin's chef”.
Born in Mr Putin’s home city of St Petersburg, Mr Prigozhin came from humble beginnings before he was able to leverage his Kremlin contracts to create a lucrative catering business.
A former convict who spent about a decade in prison for robbery in the 1980s, Mr Prigozhin started his career as a hot-dog seller after the fall of the Soviet Union.
He established a restaurant and catering business in St Petersburg in the 1990s and got to know Mr Putin, who was then the deputy mayor of a city plagued by crime.
Mr Prigozhin’s wealth took off after his company, Concord Management and Consulting, won major government contracts to provide catering services, including for schools and the military.
His prominence increased in 2014 when his newly formed Wagner Group, a private military company, helped to foment a Russia-backed uprising in eastern Ukraine.
The following year, Wagner fighters played a key role on the ground in Syria, where Mr Putin intervened with an air campaign to shore up President Bashar Al Assad in the country's civil war.
Mr Prigozhin’s infamy in the US was stoked by his indictment by federal authorities. He was accused of operating a “troll farm” that took part in Russia’s efforts to influence the 2016 US presidential election.
He expanded his reach in support of Kremlin goals to a string of African countries, stretching from Libya to the Central African Republic. He pioneered a business model based on providing security services to authoritarians in return for access to valuable natural resources, such as oil, gas and gold.
After Mr Putin invaded neighbouring Ukraine in February last year, Wagner was engaged by the Russian state to deploy tens of thousands of its combat-hardened veterans. Mr Prigozhin was even allowed to recruit from prisons, offering amnesty to those who signed up.
But the erstwhile ally of Mr Putin clashed increasingly with army chiefs, accusing them of failing to conduct the invasion aggressively enough and of depriving his fighters of supplies.
His angry public outbursts delivered on social media won him a large following in Russia.