You are not permitted to download, save or email this image. Visit image gallery to purchase the image.
Navalny (44), a prominent opponent of Russian President Vladimir Putin, started refusing food on March 31 this year in protest at what he said was the refusal of prison authorities to treat him properly for acute back and leg pain.
"His condition is indeed critical," said Alexandra Zakharova, a representative of the Doctors Alliance trade union - a group that Russian authorities regard as opposition activists.
She cited tests obtained through Navalny's lawyers, saying members of the union had not examined him themselves.
"We have seen the tests, and they are very, very bad," she told Reuters.
"His potassium is high and he has other high readings which indicate that his kidneys may soon fail. This would lead to severe pathology and cardiac arrest may occur."
Russia jailed Navalny for two-and-a-half years in February for parole violations he said were trumped up.
He was arrested at the border as he returned to Russia from Germany where he had been recovering from a nerve agent poisoning.
About 80 famous writers, actors, historians, journalists and directors, including authors JK Rowling and Salman Rushdie, wrote an open letter to Putin on Friday, urging him to ensure Navalny gets the urgent medical care he needs.
Prison authorities say they have offered Navalny proper treatment, but that he has refused it and insists he should be treated by a doctor of his choice from outside the facility, a request they have declined.
Navalny said on Friday that prison authorities had threatened to put him in a straitjacket to force-feed him unless he abandons his hunger strike.