Navalny had to be taken by ambulance overnight from Friday to Saturday to the maximum security penal colony IK-6 of Melejovo, about 250 km east of Moscow. He could not eat the prison food provided to him because it worsened his pain and since Monday he has been banned from buying alternative food.

"We cannot rule out the idea that he is being poisoned, not in a huge dose as before, but in small doses so that he does not die immediately," a spokesman said. The Kremlin said it was not monitoring the state of his health and that it was a matter for the federal prison service.