The mother of late Russian opposition leader Alexei Navalny has failed with a lawsuit alleging he had received inadequate medical care in the Arctic penal colony where he died, because he did not make the complaint himself, a Navalny ally said on Thursday.

Navalny's family and supporters have accused Russian President Vladimir Putin of having him killed, an accusation the Kremlin has angrily rejected. He had survived a poisoning with a Soviet-era nerve agent in Russia in 2020 and years of harsh treatment in prison, including long spells in solitary confinement.

Ivan Zhdanov, a former aide to Navalny, said the court in the small town of Labytnangi near the penal colony had rejected the suit on the grounds that the plaintiff must be Navalny himself.

"Alexei filed lawsuits many times for failure to provide medical care in the colonies," Zhdanov wrote on the Telegram app. "Now that he has been killed, they deny his family's claim with mocking language."

Russian prison authorities said Navalny, 47, had fallen unconscious and died on Feb. 16 after a walk outside the "Polar Wolf" prison where he was serving a three-decade sentence.

Navalny's team later said his death certificate had said he died of natural causes.

His mother Lyudmila Navalnaya travelled to the Arctic city of Salekhard, 2,000 km (1,200 miles) from Moscow, shortly after his death, where she said investigators refused to release his body from a local morgue until she agreed to bury him without a public funeral.

Authorities eventually did release Navalny's body and many thousands lined the church perimeter for his short funeral ceremony in Moscow, then filed to the cemetery to lay flowers, in defiance of Kremlin warnings against large gatherings.

(Reporting by Reuters; Writing by Lucy Papachristou; Editing by Kevin Liffey)