Mr Navalny, one of Vladimir Putin’s most prominent and persistent critics, had been behind bars since January 2021.
Rishi Sunak has paid tribute to the “fiercest advocate for Russian democracy” after the country’s prison agency said jailed opposition leader Alexei Navalny has died.
The UK Prime Minister said Mr Navalny, a prominent and persistent critic of Vladimir Putin, had “demonstrated incredible courage throughout his life”.
Moscow’s federal prison service said in a statement that Mr Navalny, 47, felt unwell after a walk on Friday and lost consciousness.
An ambulance arrived to try to rehabilitate him, but he died, the service said.
There was no immediate confirmation of Mr Navalny’s death from his team.
In a post on X, formerly Twitter, the Prime Minister said: “This is terrible news. As the fiercest advocate for Russian democracy, Alexei Navalny demonstrated incredible courage throughout his life.
“My thoughts are with his wife and the people of Russia, for whom this is a huge tragedy.”
Mr Navalny, who campaigned against official corruption and organised major anti-government protests, has been behind bars since January 2021.
He was arrested upon his return from Germany where he had recuperated from nerve agent poisoning that he blamed on the Kremlin.
Mr Navalny had rejected all charges against him as being part of a politically motivated vendetta.
Since the start of his imprisonment, Mr Navalny had continued his scathing attacks on the Kremlin in comments his associates posted to social media.
