Doctors have found that the Kremlin critic Alexei Navalny is suffering from two spinal hernias, his lawyers have said.
Vadim Kobzev told the Interfax news agency that the Russian opposition leader also has a spinal protrusion and is beginning to lose feeling in his hands.
Mr Navalny went on a hunger strike last week in protest at medical care in a Russian prison.
On Tuesday, the leader of the Navalny-backed Alliance of Doctors union was detained by police after trying to get in to the prison to talk to doctors.
Mr Navalny, 44, is the fiercest domestic opponent of Russia’s president, Vladimir Putin.
He was arrested in January on returning to Moscow from Germany, where he spent five months recovering from a nerve-agent poisoning that he blames on the Kremlin.
Russian authorities have rejected the accusation, although labs in Germany and elsewhere in Europe have confirmed Mr Navalny was poisoned with the Soviet-era Novichok nerve agent.
In February, a Russian court ordered Mr Navalny to serve two-and-a-half years in prison for violating the terms of his probation, including when he was convalescing in Germany, from a 2014 embezzlement conviction.
Mr Navalny has rejected the conviction as fabricated, and the European Court of Human Rights found it “arbitrary and manifestly unreasonable”.
Mr Navalny said in an Instagram post on Wednesday that prison authorities have tried to undermine his hunger strike by allowing aromatic chicken to be cooked in his unit’s kitchen, a departure from regulations, and by placing sweets in his clothes pockets.
“Do you know what turned out to be most important in the first stage of the prison hunger strike? Check your pockets,” he wrote.
Mr Navalny’s imprisonment has brought widespread criticism from the West.
White House spokeswoman Jen Psaki said on Wednesday that “we urge Russian authorities to take all necessary actions to ensure his safety and health”, adding that “we consider Mr Navalny’s imprisonment on trumped-up charges to be politically motivated and a gross injustice, and we stand with like-minded allies and partners in calling for his immediate release”.
Authorities transferred Mr Navalny last month from a Moscow jail to the IK-2 penal colony in the Vladimir region, 53 miles east of the Russian capital.
The facility in the town of Pokrov stands out among Russian penitentiaries for its especially strict inmate routines, which include standing at attention for hours.
Within weeks of being imprisoned, Mr Navalny said he developed severe back and leg pains and was effectively deprived of sleep because a guard checks on him hourly at night.
He went on a hunger strike on 31 March, demanding access to proper medication and a visit from his doctor.
Russia’s state penitentiary service has said Mr Navalny is receiving all the medical help he needs.
Additional reporting by agencies