The Russian opposition leader Alexei Navalny has died in jail, the country’s prison service has said, in what is likely to be seen as a political assassination attributable to Vladimir Putin.

Navalny, 47, one of Putin’s most visible and persistent critics, was being held in a jail about 40 miles north of the Arctic Circle where he had been sentenced to 19 years under a “special regime”. In a video from the prison in January, he had appeared gaunt with his head shaved.

In a statement, the federal penitentiary service for the region where Navalny was incarcerated said that he had “felt unwell after a walk and almost immediately lost consciousness”.

“All necessary resuscitation measures were carried out but did not yield positive results,” the statement read. “The paramedics confirmed the death of the convict.”

The cause of death had not been established, the penitentiary service said. Navalny had previously been treated in hospital after complaining of malnourishment and other ailments due to mistreatment in the prison.

A lawyer for Navalny did not immediately confirm reports of his death, telling the Novaya Gazeta newspaper that Navalny’s family had requested him not to comment on the reports.

“Alexei had a lawyer at his place on Wednesday,” Leonid Solovyev, his laywer, told Novaya Gazeta. “Everything was normal then.”

A close aide also did not confirm the reports of Navalny’s death.