Leading Kremlin critic Alexei Navalny plans to fly home to Russia on Sunday after recovering in Germany from his poisoning in August with a nerve agent.

Navalny announced Wednesday that he would return, despite Russian authorities' threats to put him behind bars again. He is expected to fly from Berlin to Moscow. On Thursday, Russia's prison service said that he faces immediate arrest once he returns.

Navalny, who has blamed his poisoning on the Kremlin, charged that Russian President Vladimir Putin was now trying to deter him from coming home with new legal motions. The Kremlin has repeatedly denied a role in the opposition leader’s poisoning.

FILE - In this Saturday, July 20, 2019 file photo, Russian opposition activist Alexei Navalny gestures while speaking to a crowd during a political protest in Moscow, Russia. Top Kremlin critic Alexei Navalny says he will fly home to Russia on Sunday, Jan. 17, 2021 despite the Russian prison service's intention to put him behind bars for allegedly breaching the terms of his suspended sentence and probation.(AP PhotoPavel Golovkin, File)

FILE - In this Saturday, July 20, 2019 file photo, Russian opposition activist Alexei Navalny gestures while speaking to a crowd during a political protest in Moscow, Russia. Top Kremlin critic Alexei Navalny says he will fly home to Russia on Sunday, Jan. 17, 2021 despite the Russian prison service's intention to put him behind bars for allegedly breaching the terms of his suspended sentence and probation.(AP PhotoPavel Golovkin, File)

At the end of December, the Federal Penitentiary Service, or FSIN, warned Navalny that he faced time in prison if he fails to immediately report to its office in line with the terms of a suspended sentence and probation he received for a 2014 conviction on charges of embezzlement and money laundering that he rejected as politically motivated. The European Court for Human Rights had ruled that his conviction was unlawful.

The FSIN said Thursday it issued an arrest warrant for Navalny after he failed to report to its office. The prison service, which has asked a Moscow court to turn Navalny’s 3 1/2-year suspended sentence into a real one, said it’s “obliged to take all the necessary action to detain Navalny pending the court’s ruling.”

Navalny fell into a coma while aboard a domestic flight from Siberia to Moscow on Aug. 20. He was transferred from a hospital in Siberia to a Berlin hospital two days later.

In this handout photo taken from a video released on Wednesday, Jan. 13, 2021, by Russian opposition activist Alexei Navalny in his instagram account, Russian opposition activist Alexei Navalny gestures as he records his address. Top Kremlin critic Alexei Navalny says he will fly home to Russia over the weekend despite the Russian prison service's latest motion to put him behind bars for allegedly breaching the terms of his suspended sentence and probation. (Navalny instagram account via AP)

In this handout photo taken from a video released on Wednesday, Jan. 13, 2021, by Russian opposition activist Alexei Navalny in his instagram account, Russian opposition activist Alexei Navalny gestures as he records his address. Top Kremlin critic Alexei Navalny says he will fly home to Russia over the weekend despite the Russian prison service's latest motion to put him behind bars for allegedly breaching the terms of his suspended sentence and probation. (Navalny instagram account via AP)

Labs in Germany, France and Sweden, and tests by the Organization for the Prohibition of Chemical Weapons, established that he was exposed to a Soviet-era Novichok nerve agent.

Russian authorities insisted that the doctors who treated Navalny in Siberia before he was airlifted to Germany found no traces of poison and have challenged German officials to provide proof of his poisoning. They refused to open a full-fledged criminal inquiry, citing a lack of evidence that Navalny was poisoned.

Last month, Navalny released the recording of a phone call he said he made to a man he described as an alleged member of a group of officers of the Federal Security Service, or FSB, who purportedly poisoned him in August and then tried to cover it up. The FSB dismissed the recording as fake.