Russia’s prison service on Monday gave Kremlin critic Alexei Navalny a last minute ultimatum: Fly back from Germany at once and report at a Moscow office early on Tuesday morning, or be jailed if you return after that deadline.

Navalny, one of President Vladimir Putin‘s leading critics, was airlifted to Germany for treatment in August after collapsing on a plane in what Germany and other Western nations say was an attempt to murder him with a Novichok nerve agent.

Russia has said it has seen no evidence he was poisoned and has denied any involvement in the incident.

The Federal Prison Service (FSIN) on Monday accused Navalny of violating the terms of a suspended prison sentence he is still serving out over a conviction dating from 2014, and of evading the supervision of Russia’s criminal inspection authority.

Citing an article in the British medical publication The Lancet about his treatment, it said Navalny had been discharged from hospital in Berlin on Sept. 20 and that all symptoms of what it called his illness had vanished by Oct. 12.

Read more: Reuters