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, or be jailed if you return after that deadline, writes Reuters.

The Federal Prison Service 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.

The prison service said in a statement late on Monday that it had summoned Navalny to report to the inspection authority and that his suspended sentence could be changed to a real jail term if his suspected violations of the terms of the suspended sentence were proven to be true.

The prison service mentioned no deadline, but Navalny posted a screenshot of a message to his lawyer which said he had until 09:00 on Tuesday to return and show up at a Moscow office.