Russian investigators have yet to determine the official cause of opposition leader Alexei Navalny’s death, his spokeswoman says, after prison authorities said he suddenly dropped dead following a walk in the “Polar Wolf” penal colony.

The death of Navalny, a former lawyer, robs the disparate Russian opposition of its most charismatic and courageous leader as President Vladimir Putin prepares for an election that will keep him in power until at least 2030.

The West and Navalny’s supporters have dismissed the Russian prison service’s statement on Friday that the 47-year-old had lost consciousness and died.

Western leaders have cast Putin as a killer and said he is responsible for Navalny’s death.

Putin has made no public comment but Russian officials scolded Western leaders for rushing to conclusions about the death, which has further deepened a schism in relations between Moscow and the West caused by the Ukraine war.

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Navalny’s mother, Lyudmila, was told on Saturday at the prison colony that he had perished from “sudden death syndrome”, a vague term for different heart conditions that end in death, according to Navalny’s team.

Navalny’s spokeswoman, Kira Yarmysh, said his 69-year-old mother and lawyers were informed that the official verification of the cause of death had been extended and that it was unclear how long it would take.

“The cause of death is ‘undetermined’,” said Yarmysh, who accused the Russian authorities were lying and stalling.

His mother and lawyers were not allowed into the morgue on Monday in the Arctic town near the prison colony where the authorities said he dropped dead, Yarmysh said.

“Asked if Alexei’s body was there, the staff did not answer,” Yarmysh said.

Navalny had been incarcerated at the IK-3 penal colony north of the Arctic Circle located in Kharp in the Yamalо-Nenets region about 1900 kilometres north-east of Moscow.

Just the day before his death, Navalny was shown joking in court via a video link to court.

He teased the judge, suggesting he use some of his vast salary to top up Navalny’s own account.

Alexei Navalny at Khimki in January 2021, shortly after returning to Russia. Photo: AFP/Getty

Navalny’s death came just as Russian forces made their biggest advance in the Ukraine war since May 2023 and as the West debates how much more support to give to Kyiv after a counteroffensive last year failed to pierce Russian lines.

US President Joe Biden blamed Putin for Navalny’s death and warned there could be consequences.

However, it is unclear what more the West can do after imposing on Russia what it says are already the toughest sanctions ever imposed on a major economy over the Ukraine war.

Kremlin spokesman Dmitry Peskov the West’s reaction to Alexei Navalny’s death was unacceptable but the “frankly obnoxious” statements from the United States and Europe would not harm Putin.

However, “these statements, of course, cannot cause any damage to our head of our state”, he said.

Peskov said the investigation into Navalny’s death was ongoing and was being conducted in accordance with Russian law.

Foreign ministry spokeswoman Maria Zakharova said the reaction of the West and its media demonstrated “their hypocrisy, cynicism and unscrupulousness”.

“In any situation, the script is always: ‘blame Russia’,” Zakharova said.