Alexei Navalny, the chief foe of Vladimir Putin’s regime, has died in prison.

The prominent Russian opposition politician “felt ill” on Friday after a walk, “almost immediately lost consciousness” and died soon after, prison authorities said.

State news agency Tass reported that doctors were unable to resuscitate Mr Navalny, and his cause of death is “being established”.

Charles Michel, president of the European Council, said the European Union holds Russia “solely responsible” for his death.

Volodymyr Zelensky said it was “obvious” that Putin was behind it “like other thousands who were tortured to death because of this one man”.

Joe Biden, the US president, said in June 2021 that the consequences of Mr Navalny’s death “would be devastating for Russia”.

The Kremlin said it had “no information” on Mr Navalny’s death.

Mr Navalny previously survived an attempted assassination in August 2020 when he was poisoned with the Novichok nerve agent on an internal flight.

He had emerged as Russia’s most prominent opposition politician and led significant street protests against corruption and Putin’s regime.

Nato secretary-general Jens Stoltenberg said Russia must be made to answer “all the serious questions about the circumstances of his death”.

Rishi Sunak described Mr Navalny’s death as a “huge tragedy” for “the people of Russia”.

“As the fiercest advocate for Russian democracy, Alexei Navalny demonstrated incredible courage throughout his life,” he said.

Edgars Rinkevics, president of Latvia, said Mr Navalny had been “brutally murdered by the Kremlin”.

Dmitry Gudkov, the exiled Russian opposition politician, added: “Even if Alexei died of ‘natural causes’, they were triggered by his poisoning and the ensuing torture in prison. (Navalny’s) blood is on Putin’s hands.”

Mr Navalny was “fine” when a lawyer visited him at the prison on Wednesday, his leading legal counsel, Leonid Solovyov, told the Russian newspaper Novaya Gazeta.

The newspaper published footage of him appearing in court on Thursday via video link, showing him “healthy and cheerful”.

Leonid Volkov, Mr Navalny’s former campaign manager and one of his closest advisers, said a lawyer was on his way to the remote prison colony where he was being held in Yamal, western Siberia, to confirm the reports of his death.

Kira Yarmysh, Mr Navalny’s spokesperson, said there was currently “no confirmation”.

He was jailed for two-and-a-half years in 2021 after returning to Russia upon his recovery from the attempted poisoning. This sentence was extended by nine years in March 2022 on embezzlement and contempt of court charges.

A court then extended his sentence to 19 years in August last year on extremism charges and moved him to a harsher prison.

In December, Mr Navalny went missing from his prison colony for two weeks, later re-appearing at the “Polar Wolf” colony in the Arctic Circle,, considered one of the country’s toughest jails.

Dmitry Muratov, the Russian newspaper editor and Nobel Peace Prize laureate, told Reuters that he believed the death was “murder” brought on by the conditions Mr Navalny was being held in.

Russia will hold a presidential vote in March, expected to hand Putin a fifth term as president.

Moscow has for years sidelined opposition figures, a clampdown that intensified after the Kremlin ordered Russian troops into Ukraine in 2022.

Boris Nadezhdin, a vocal war opponent, was disqualified from standing in March’s election despite earning the 100,000 signatures necessary to get on the ballot.