Russian opposition leader Alexei Navalny has started a prison hunger strike to protest over officials’ failure to provide proper treatment for his back and leg pains.

In a statement posted on Instagram, he complained about prison authorities’ refusal to give him the right medicines and to allow his doctor to visit him behind bars.

He also protested against hourly checks a guard makes on him at night, saying they amount to sleep deprivation torture.

Mr Navalny said in his statement that he had no choice but to protest with a hunger strike because his physical condition has worsened. He said his back pains have spread to his right leg and he feels numbness in his left leg.

“What else could I do?” he wrote. “I have declared a hunger strike demanding that they allow a visit by an invited doctor. So I’m lying here, hungry, but still with two legs.”

The 44-year-old, who is President Vladimir Putin’s most outspoken opponent, was arrested in January on his return 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.

Last month, he was sentenced to two and a half years in prison for violating the terms of his probation during his convalescence in Germany.

The sentence stems from a 2014 embezzlement conviction he has rejected as fabricated, and which the European Сourt of Human Rights has ruled to be unlawful.

Mr Navalny was moved this month from a Moscow jail to a prison colony in Pokrov in the Vladimir region, 50 miles east of the Russian capital.

The facility stands out among Russian penitentiaries for its particularly strict inmate routines, which include standing at attention for hours.

Mr Navalny’s Instagram had a picture of a letter to the prison chief, dated Wednesday, in which he declared the hunger strike.

“Every convict has the right to invite a specialist for a check and consultation,” he wrote. “So I demand to let a doctor see me and declare a hunger strike until it happens.”

Russia’s Federal Penitentiary Service said last week that Mr Navalny had undergone medical check-ups and described his condition as “stable and satisfactory”.

But he complained that authorities only gave him basic painkillers and ointment for his back and legs while refusing to accept medications prescribed earlier by his doctor or to share the diagnosis from his examination.

In a note earlier this month, Mr Navalny described his prison as a “friendly concentration camp”. He said he had not seen “even a hint at violence” there but lived under controls that he compared to George Orwell’s Nineteen Eighty-Four.

Mr Navalny, whom prison authorities had earlier marked as a flight risk, said he was subject to particularly close oversight, including a guard waking him up every hour at night and filming him to demonstrate he is in the required place.

“Instead of medical assistance, I’m subjected to sleep deprivation torture, being woken up eight times every night,” he said in Wednesday’s statement.

During a video call with Mr Putin on Tuesday, German Chancellor Angela Merkel and French President Emmanuel Macron emphasised the need for Russia to protect Mr Navalny’s health and to respect his rights in compliance with the European Convention on Human Rights, according to Mr Macron’s office.

The Kremlin said in its readout of the call that Mr Putin offered an “objective explanation” in response to questions about Mr Navalny.