Alexei Navalny, a Russian Opposition politician and prominent Kremlin critic, died in prison on Friday, Reuters reported citing the country’s prison service.

The Federal Penitentiary Service of the Yamalo-Nenets Autonomous District reportedly stated that Navalny “felt unwell” after a walk and lost consciousness.

“The medical staff of the institution arrived immediately, and an ambulance team was called,” the prison service said. “All necessary resuscitation measures were carried out, which did not yield positive results. Doctors of the ambulance stated the death of the convict.”

The prison service stated that the cause of the death was “being established”.

Navalny’s spokesperson Kira Yarmysh said in a social media post that his team had no confirmation of his death and his lawyer was traveling to the town where he was held.

Navalny had campaigned to run against Putin in the 2018 presidential elections but was barred from contesting. Navalny’s Foundation for Fighting Corruption had exposed graft among high-level government officials. However, in 2020, the Opposition leader had to shut down the organisation after Yevgeny Prigozin, a businessman with close ties to the Kremlin, filed a lawsuit against him.

The same year, Navalny was poisoned by what German military tests showed was a Soviet-style Novichok chemical nerve agent. The findings were corroborated by labs in France and Sweden.

Novichok, a potent class of chemical weapon developed by the Soviet Union, is the same substance that was used on former spy Sergei Skripal and his daughter in England in 2018.

For five months, Navalny stayed in Germany to recover. After the activist announced his return in January 2021, the Moscow prison service said it would “do everything to arrest him once he returned”.

On January 17, 2021, Navalny was arrested on arrival in Moscow on charges of flouting the terms of a suspended sentence for embezzlement. While he maintained that the charges against him were false and trumped up, he was sentenced to 2.5 years in prison.

In March 2022, Navalny was sentenced to nine years in a maximum security penal colony on charges of large-scale fraud and contempt of court.

A Russian court in August convicted Navalny on charges of extremism and sentenced him to 19 years in prison. He was to serve the new term concurrently with his older sentences.

“I understand perfectly that, as many political prisoners, I’m serving a life sentence, which is measured by the length of my life or the length of life of this regime,” Navalny had said after the sentence.