The 44-year-old Navalny, President Vladimir Putin's most visible and persistent critic, started a hunger strike more than three weeks ago to protest prison authorities' refusal to allow him to be seen by a private doctor for diagnosis of severe back pain and loss of feeling in his legs; the Russian penitentiary service says he is getting adequate care. Navalny blames the poisoning on the Kremlin.
Prison authorities say they have offered Navalny proper medical care but that the 44-year-old opposition politician has refused it and insisted on being treated by a doctor of his choice from outside the facility, a request they have declined.
Anastasia Vasilyeva, Navalny's personal physician, said in the letter that the blood tests showed his potassium had reached the point where kidney failure and cardiac arrest "can happen any minute".
"Fatal arrhythmia can develop any minute".
The letter raised similar health concerns to those voiced on Tuesday by Navalny's wife, who said after visiting him in prison that he was having difficulty speaking and had lost more weight.
She cited tests obtained through Navalny's lawyers, saying members of the union had not examined him themselves.
Alexei Navalny, 44, almost died in August 2020 when he was poisoned with the chemical nerve agent, Novichock.
She said she felt like she was "on that plane again, only this time it's landing in slow motion", pointing out that access to Navalny was restricted and few Russians were aware of what was actually going on with him in prison.
Navalny, who the West says has been wrongly jailed and should be freed, returned to Russian Federation in January after recovering from what German doctors say was a nerve agent poisoning.
More than 70 prominent global writers, artists and academics, including Jude Law, Vanessa Redgrave and Benedict Cumberbatch, have called on Putin to ensure that Navalny receives proper treatment immediately. Police arrested more than 10,000 people during nationwide protests in January calling for Navalny to be freed.
Earlier this week, Navalny's wife Yulia, who visited him in the penal colony, said her husband now weighed 168 pounds - down almost 20 pounds since starting his hunger strike.
The Russian Public Prosecution demanded Friday that the network of regional offices of Navalny and his anti-corruption institution be classified as "extremist organizations".