Navalny, who has been sentenced to nearly three decades in prison, disappeared from a prison in the Vladimir region near Moscow last week, and was suspected to have been transferred into a “special regime” colony, where he could be held incommunicado under Russia’s harshest prison regime for years.

Kira Yarmysh, an aide to Navalny, said his team had sent requests to nearly 200 Russian pre-trial detention centres searching for more information on the missing opposition leader but had not been able to find him.