Navalny, a fierce critic of President Vladimir Putin and his lieutenants, is in serious condition after drinking tea on Thursday morning that his allies believe was laced with poison.

Doctors treating him in Omsk, Siberia, said his condition had improved a little overnight but his life was still in danger.

Alexander Murakhovsky, the head doctor at the hospital, said Navalny had been diagnosed with a metabolic disease that may have been caused by low blood sugar.

He said traces of industrial chemical substances had been found on the 44-year-old’s clothes and fingers hours after the hospital said doctors did not believe Navalny had been poisoned.

Navalny’s wife Yulia and his spokeswoman Kira Yarmysh, who want to fly Navalny to Germany for medical treatment, have criticised the hospital after it said that moving him would put his life at risk because he was still in a coma and his condition unstable.

“The ban on transporting Navalny is an attempt on his life being carried out right now by doctors and the deceitful authorities that have authorised it,” Yarmysh wrote on Twitter.

She said doctors had previously consented to his being moved, but had withheld their agreement at the last minute.

“This decision, of course, was not made by them, but by the Kremlin,” said Yarmysh.

The Kremlin said on Friday it was up to doctors to decide whether Navalny was fit to be moved from the hospital.

The row broke out as an air ambulance sent by the Berlin-based Cinema for Peace Foundation, a non-profit organisation, landed in Omsk with the intention of flying him to Germany if possible.

Murakhovsky told reporters that many legal questions would need to be resolved before Navalny could be handed over to European doctors.

He said top doctors had been flown in from Moscow to treat Navalny who were no worse than their European counterparts. Test results would be available within two days, he said.

Navalny has been the biggest thorn in the Kremlin’s side for more than a decade, exposing what he says is high-level graft and mobilising crowds of young protesters.

He has been repeatedly detained for organising public meetings and rallies and sued over his investigations into corruption. He was barred from running in a presidential election in 2018.

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Navalny’s team cited a police officer as saying a highly dangerous substance had been identified in his body that posed a risk to everyone around him and that they should wear protective suits. Reuters could not independently confirm that information.

Navalny’s team said it believed authorities wanted to stall for time so that any trace of what poisoned him would disappear.

French junior European Affairs Minister Clement Beaune said on Friday he shared concerns and fears about Navalny’s fate, and reiterated that France was ready to help him if necessary. (Reporting by Anton Zverev in Omsk, Gabrielle Tétrault-Farber, Andrey Kuzmin and Tom Balmforth in Moscow; Writing by Andrew Osborn/Tom Balmforth Editing by Nick Tattersall and Angus MacSwan)