Russia's prison authorities, which have barred Mr. Navalny's own medical team from visiting him, said its doctors had made a decision to move him to a medical facility on the premises of another penal colony outside Moscow.
Navalny was arrested in Russian Federation in January after returning from treatment in Germany for the nerve agent poisoning in August he said was carried out by Moscow - accusations denied by Putin's administration.
But the opposition leader's physician, Dr. Yaroslav Ashikhmin, said Saturday that test results provided by the family show Navalny has sharply elevated levels of potassium, which can bring on cardiac arrest, and heightened creatinine levels that indicate impaired kidneys.
Russian opposition figure Alexei Navalny should immediately be granted access to proper medical care while imprisoned at a penal colony outside Moscow, European Commission President Ursula von der Leyen said Sunday amid reports that his health had sharply deteriorated.
"We are talking now only about health and life".
Local prison authorities said in a statement on Monday that Mr Navalny's condition was "satisfactory" and that he had been taken to a hospital ward in another prison. He was detained and incarcerated for a six-year-old conviction that was found unlawful by the European Court of Human Rights.
Mr. Navalny on March 31 went on a hunger strike for proper treatment.
"With the patient's consent, he was prescribed vitamin therapy", Russia's federal penitentiary service said, per The Washington Post.
"The health of convicts in the Russia Federation can not and should not be a topic concerning them", said Mr. Putin's spokesman Dmitry Peskov.
After news of his transfer to a prison hospital was announced, top Navalny strategist Leonid Volkov dismissed this step, per The Associated Press, saying, "Until the lawyers locate him, we won't know where he is and what is up with him".
Ivan Zhdanov, the head of Navalny's Foundation for Fighting Corruption, tweeted Monday that the transfer would take the politician merely to another "tormenting colony, just with a big in-patient facility, where gravely ill are being transferred". He said he had been searched for two hours on arrival at the penal colony.
Navalny has complained about being sleep-deprived because guards conduct hourly checks on him at night, and said he developed severe back pain and numbness in his legs within weeks of being transferred to the colony. "On the basis of having the poison and then on a hunger strike".
In a message from prison on Friday, Navalny said prison officials threatened to force-feed him "imminently", using "straitjacket and other pleasures".
"We urgently demand that Alexey Navalny receive adequate medical treatment and access to doctors he trusts", Maas told Bild.
Navalny allies have said they are bracing for bad news and that they are planning nationwide protest rallies to call for this release.