Recent photographs have shown the prisoner looking frail in his cell and more than 500 Russian doctors have written to the Russian president to ask for the “abuse” to end.
While the 47-year-old has been seen by a doctor, he is not receiving the medical treatment he needs and has even been denied basic medicine. He is said to be suffering from the chills, a fever, severe cough and a low temperature.
In the letter to the Russian president, the doctors expressed their "great concerns for the life and health" of Mr Navalny and said the denial of basic medicine is “directly threatening” his life.
The German government has also called for Mr Navalny to be allowed medical attention.
Christiane Hoffmann, a German government spokesperson, has reportedly said he “is in urgent need of medical assistance as numerous Russian doctors have pointed out”.
“We call on the Russian authorities to provide this immediately and completely,” she added.
Amnesty International has called the legal proceedings against Mr Navalny as a “sham” and his imprisonment led to protests across Russia.
Mr Navalny has campaigned against corruption within the Kremlin and was put behind bars in January 2021 - after returning from Germany, where he had been treated for a nerve agent attack. He has led the Russia of the Future political party since 2013 - the same year he was arrested for embezzlement and then released as part of a suspended sentence.
It was ruled in 2021 that his trip to Germany broke his parole conditions and he was sentenced to two and a half years behind bars.
Last year, a further nine years were added to his sentence after being found guilty of embezzlement and contempt of court.
Mr Navalny has described being taken to the prison’s punishment cell more than ten times since he was locked up. He also said guards put him with an inmate suffering from flu so that the virus could transfer onto him.