Jailed Russian opposition politician Aleksei Navalny says he has been moved back into "extremely hellish" solitary confinement just three days after finishing his latest 15-day sentence in the punishment cell.

Navalny said he was sentenced to another 15 days in the SHIZO, a Russian abbreviation for punishment cell on April 11, the day after his supporters released an investigation into the Russian penitentiary service’s purchase of cabbage, suggesting that corrupt prison authorities pay exorbitant prices and keep a cut of the money for themselves.

“The wholesale price of cabbage in the summer was 6 rubles per kilo. Meanwhile, the Federal Penitentiary Service buys it in unthinkable quantities for 27.50 rubles per kilo,” Navalny said on Twitter, adding that there are 400,000 convicts in the country, and cabbage is their main ration.

“Everyone gets their cut here: from the head of the Federal Penitentiary Service Gostev, who meticulously ignores the insane prices, to the management of the [prison] colonies,” he said, referring to Arkady Gostev, whom President Vladimir Putin appointed to head the Federal Penitentiary Service in November 2021.

“Putin's entire system only exists in order to steal money on cabbage,” Navalny said, adding that the same practices apply to other government purchases. “The system will steal at all levels, because that is its whole point.”

Navalny, 46, is serving a 2 1/2-year prison sentence for violating the terms of an earlier parole when he was medically evacuated in August 2020 from Siberia in a coma after suffering the near-fatal poisoning that he blames on Russian security operatives acting at Putin's behest.

The prison sentence is widely considered to be politically motivated.

Navalny returned to Russia in January last year and in March was handed a separate nine-year prison term on charges of contempt and embezzlement through fraud that he and his supporters have repeatedly rejected as politically motivated.

The anti-corruption campaigner’s team said last month shortly after a documentary film about Navalny won an Academy Award that he had spent more than 100 days of the previous six months in the punishment cell.

Navalny said on Twitter that he lost 8 kilos during the most recent 15-day stretch in solitary confinement, and his supporters say his health has worsened.

An ambulance was called to the colony on April 7 after Navalny's condition deteriorated as the result of a stomach ailment, said Vadim Kobzev, one of Navalny's lawyers.

Navalny also said prison authorities set up a “personal sewing room” where he can work in the adjoining cell and implied that it is just as small as the SHIZO. But he claimed prison officials have made up “completely unlawful rules” that forbid him from buying food with the money he earns.

The Russian Interior Ministry did not respond to a request for comment, Reuters said.

With reporting by Reuters