MOSCOW — Aleksei A. Navalny, the imprisoned opposition leader who returned to Russia this year despite an earlier attempt on his life, is in deteriorating health with unexplained ailments and has received substandard medical care, his lawyers said on Thursday.
Prison doctors moved Mr. Navalny, 44, to a hospital for tests on Wednesday but offered no explanation for his complaints of severe back pain and numbness in one leg, and then returned him to the penal colony east of Moscow, said his lawyer, Olga Mikhailova.
“His health is extremely unfavorable, and every day gets worse,” she said in an interview after meeting Mr. Navalny in prison on Thursday. His right leg has numbed to the point that he cannot put weight on it, she said. “We are afraid for his life and his health.”
Mr. Navalny collapsed into a coma on an airplane flight last August and was medically evacuated from Russia to Berlin. After extensive tests there, both the German and French governments, and international chemical weapons specialists, confirmed that he had been poisoned with a Soviet-designed military nerve agent, Novichok.
The poisoning was the latest in a series of assassinations and attempted assassinations of opponents of President Vladimir V. Putin that Western governments have blamed on the Kremlin. Mr. Putin has denied any state role in Mr. Navalny’s collapse, arguing that if Russian agents had wanted to kill him, they would have succeeded.
Mr. Navalny returned to Russia in January despite his supporters’ fears for his safety. He was detained for a parole violation — failure to report to Russian authorities while he was being treated in Germany for the poisoning — on a previous offense that he and his allies dismiss as politically motivated, and sentenced to more than two years in a penal colony.
His new, so far undiagnosed symptoms, including back pain, began a month ago but worsened this week, and he could not rule out lingering effects of the nerve agent poisoning, Ms. Mikhailova said.
Mr. Navalny had asked his lawyers not to make the symptoms public before this week, she said. Only after prison officials on Wednesday declined to allow a meeting with Mr. Navalny did they air their worries, which she said deepened on Thursday after a meeting was granted.
Prison doctors have provided only ibuprofen pills and an ibuprofen-based topical ointment for pain, she said, and have refused to pass along medicines the lawyers provided or allow access for a personal doctor.
The prison health system, she said, has not provided the lawyers or Mr. Navalny a diagnosis for the symptoms. She said she feared that if Mr. Navalny is not transferred to receive specialty care soon, his condition will deteriorate.
Earlier on Thursday, Russia’s prison authorities said Mr. Navalny’s health was “stable and satisfactory” after an examination, the Tass news agency reported.
Prison doctors are not qualified to treat him, Ms. Mikhailova said, and she has filed appeals to move him to Moscow to be examined by a specialist. “His condition is worsening, not improving, with the treatment he is getting in prison,” she added.
His condition has been exacerbated by sleep deprivation, she said, with guards awakening him hourly, ostensibly to confirm his presence in the prison barracks, as he is classified as a flight risk.
After his poisoning, Mr. Navalny and the open-source investigative group Bellingcat studied phone records of Russian security service agents and other clues to reconstruct what they called an attempted assassination.
Mr. Navalny said the poison, which can be lethal to the touch, had been applied to the inside of a pair of his underpants. It was the same class of nerve agent that sickened several people in England and killed one of them in 2018, in what western intelligence agencies said was a failed attempt to kill Sergei V. Skripal, a former Russian spy.
In Germany, Mr. Navalny underwent months or rehabilitation and in interviews described harrowing neurological symptoms, including disorientation and trouble walking. By late last year, he said he had seemed to fully recover.