Russian opposition leader Alexei Navalny who is in the third week of a hunger strike, will be admitted to a hospital in another prison, the Russian state penitentiary service said.

The move comes after the politician’s doctor said he could be near death.

The prison service, FSIN, also said Mr Navalny had agreed to take vitamin therapy, but an ally of the 44-year-old Kremlin critic cast doubt on that and the hospital transfer, saying his lawyers should confirm both.

The service said in a statement that Mr Navalny would be transferred to a hospital for convicts located in a penal colony in Vladimir, a city 180 kilometres (110 miles) east of Moscow.

According to the statement, Mr Navalny’s condition is deemed “satisfactory”.

But the opposition leader’s doctor, Yaroslav Ashikhmin, said on Saturday that test results he received from the family show him with sharply elevated levels of potassium, which can bring on cardiac arrest, and heightened creatinine levels that indicate impaired kidneys.

“Our patient could die at any moment,” he said in a Facebook post.

Mr Navalny, President Vladimir Putin’s fiercest opponent, was arrested in January upon his return from Germany, where he had spent five months recovering from a nerve agent poisoning he blames on the Kremlin – accusations Russian officials have rejected.

Mr Navalny’s arrest triggered a massive wave of protests all across Russia, the biggest show of defiance in recent years.

Soon after, a court ordered Mr Navalny to serve two-and-a-half years in prison on a 2014 embezzlement conviction that the European Court of Human Rights deemed to be “arbitrary and manifestly unreasonable”.

Mr Navalny went on hunger strike in prison to protest over the refusal to let his doctors visit when he began experiencing severe back pain and a loss of feeling in his legs.

Russia’s state penitentiary service has said that Mr Navalny was receiving all the medical help he needs.

In response to the alarming news about Mr Navalny’s health this weekend, his team has called for a nationwide rally on Wednesday, the same day that Mr Putin is scheduled to deliver his annual state of the nation address.

Several Navalny allies dismissed the move announced by the prison service as insufficient.

Mr Navalny’s top strategist, Leonid Volkov, said no-one should assume it was happening until the opposition leader’s lawyers confirm it.

The lawyers were en route to the prison where the hospital was located, Mr Volkov said.

“Until the lawyers locate him, we won’t know where he is and what is up with him,” Mr Volkov wrote in a Facebook post.

Ivan Zhdanov, the head of Mr Navalny’s Foundation for Fighting Corruption, tweeted that the move announced by the prison service would take the politician merely to another “tormenting colony, just with a big in-patient facility, where gravely ill are being transferred”.

Dr Anastasia Vasilyeva, head of the Navalny-backed Alliance of Doctors union and also the politician’s personal doctor, noted that it was “not a hospital where a diagnosis can be determined and treatment (can be) prescribed for his ailments”, but rather “a prison where tuberculosis is being treated”.

She again called for the prison to let her and other doctors see him.

Last month, the politician was transferred to a penal colony east of Moscow, notorious for its harsh conditions.

Mr Navalny has complained about being sleep-deprived due to guards conducting hourly checks on him at night, and said he has developed severe back pain and numbness in his legs within weeks of being transferred to the colony.

His demands for a visit from an independent “civilian” doctor were rebuffed by prison officials, and he went on hunger strike on March 31.

In a message from prison on Friday, Mr Navalny said prison officials threatened to force-feed him “imminently”, using “straitjacket and other pleasures”.