MOSCOW — The health of Aleksei A. Navalny, the imprisoned Russian opposition leader who last year survived a nerve agent poisoning, has deteriorated rapidly in recent days with undiagnosed back pain and numbness in one leg, his lawyers said Thursday.

Prison doctors moved Mr. Navalny, who is 44 years old, to a hospital for tests on Wednesday but by evening had returned him to the penal colony east of Moscow where he is serving a sentence of more than two years, without providing an explanation for his symptoms, his lawyer, Olga Mikhailova, told the Russian news media.

“His health condition is extremely unfavorable,” Ms. Mikhailova said after meeting with Mr. Navalny on Thursday. “He is experiencing strong pain in his back” and his right leg has numbed to the point he cannot walk on it, she said. “His right leg is in a terrible state.”

The symptoms began a month ago, she said, but Mr. Navalny had asked the lawyers not to make them public. Only after prison officials on Wednesday declined to allow a meeting with Mr. Navalny did they make their concerns public. “We became afraid for his life and health,” Ms. Mikhailova said.

Prison doctors have provided only ibuprofen and an ibuprofen-based topical ointment for pain, she said, and have refused to pass along medicines the lawyers provided or to allow a personal doctor to examine him.

The prison health system, she said in an interview with Dozhd television, an independent Russian news outlet, has not yet made a diagnosis. She said she feared that if Mr. Navalny is not transferred to specialty care soon, he could “leave here as an invalid.”

Earlier on Thursday, Russia’s prison authorities said Mr. Navalny’s health was “deemed stable and satisfactory” after an examination, the Interfax news agency reported.

Mr. Navalny collapsed into a coma on an airplane flight last August and was medically evacuated to a hospital in 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.

Mr. Navalny underwent months or rehabilitation, and in interviews described harrowing neurological symptoms including disorientation and trouble walking. But by late last year he said he had seemed to have made a full recovery.

Mr. Navalny and Western security agencies blamed the Russian government for what they said was an assassination attempt. Russian President Vladimir V. Putin has denied any role in Mr. Navalny’s collapse last summer, arguing that if Russian agents had wanted to kill him they would have succeeded.

Mr. Navalny returned to Russia in January and was arrested upon arrival for a supposed parole violation for an original offense that he has always dismissed as politically motivated. The parole violation was failure to report to Russian authorities while he was recovering in Berlin from the poisoning.