President Vladimir Putin’s chief critic, Aleksei Navalny, imprisoned for his outspoken opposition against the Russian leader, is dead.

Russian prison authorities issued a statement announcing the demise of Mr Navalny, a tough critic of President Vladimir Putin, who was serving a 19-year sentence for mostly political offences.

Mr Navalny, 47, had “felt unwell” after taking a walk and “immediately lost consciousness,” Russia’s Federal Penitentiary Service said in a statement Friday. “The facility’s medical staff immediately arrived and an ambulance brigade was called.”

The penitentiary disclosed that all efforts made to revive Mr Navalny by healthcare personnel failed, leading doctors to declare him dead.

“All necessary resuscitation measures were taken, which did not lead to positive results. The ambulance doctors confirmed the death,” the statement said, adding that the prominent activist was a “convict”.

But Mr Navalny’s spokesperson, Kira Yarmysh, stated in a social media post that she was unable to confirm the report of his death but that a lawyer was on the way to the prison to gather details.

“We don’t have any confirmation of this yet,” Yarmysh said. “As soon as we have any information, we will report it.”

Mr Navalny’s sudden demise mirrors that of Yevgeniy Prigozhin, the head of the Wagner mercenaries, who perished in a plane crash that killed all 10 people on board, just months after opposing the Russian president last August.