Russia’s prison service  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.

“Therefore the convicted man is not fulfilling all of the obligations placed on him by the court, and is evading the supervision of the Criminal Inspectorate,” it said.

Navalny is serving out a suspended three-and-a-half-year prison term over a theft case he says was politically-motivated. His probation period expires on Dec. 30.

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 0900 on Tuesday to return and show up at a Moscow office.

His spokeswoman, Kira Yarmysh, said on Twitter, it was impossible for Navalny to return in time, that he was still convalescing after his poisoning, and accused the prison service of acting on orders from the Kremlin.

“There’s no way he could appear at the Moscow Criminal Inspectorate tomorrow. But does the FSIN really care about common sense? They were given an order, they are fulfilling it,” she wrote.

The Kremlin has said Navalny is free to return to Russia at any time like any other Russian citizen.