A prominent critic of Vladimir Putin has arrived in Germany for medical treatment, amid claims he became seriously ill after drinking a cup of tea laced with poison.

Alexei Navalny, who remains in a coma after his condition deteriorated on Thursday, was transported from a Siberian hospital to Berlin on a special medical plane.

The Russian opposition leader’s supporters claim that the Kremlin is responsible for his illness, as well as the delay in transferring him overseas.

Doctors in the Siberian city of Omsk have rejected the possibility that Mr Navalny was poisoned – suggesting a drop in blood sugar may have caused him to lose consciousness.

Alexei Navaly is transferred to an ambulance from Omsk hospital in Siberia and being driven to airport, his spokeswoman, Kira Yarmysh, said on Twitter
Image: An ambulance took Navalny from Omsk hospital to the airport, his spokeswoman said

But Western toxicology experts have expressed doubts over whether a poisoning could be ruled out so quickly.

Early on Saturday morning, Mr Navalny’s spokeswoman Kira Yarmysh tweeted: “The plane with Alexei has taken off for Berlin. A huge thank you to everyone for your support.

“The struggle for Alexei’s life and health is just beginning and there will be much more to go through, but at least now the first step has been taken.”

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Ms Yarmysh said Navalny’s wife, Yulia, who has been campaigning for his removal from the Siberian hospital was also with him on board.

Flight tracking data showed the plane safely landed at Tegel airport several hours later.

There was an extensive police presence outside Charite hospital, a clinic in Berlin that has a history of treating foreign leaders and dissidents.

Mr Navalny’s arrival in Berlin was punctuated by delays. Initially, his doctors in Siberia had said he was too unstable to be moved, despite a team of German specialists being dispatched to move him.

The 44-year-old’s supporters said the doctors were stalling until any poison in his system would no longer be traceable.

Leaders of France and Germany said on Thursday they were ready to assist Mr Navalny and his family, and called for an investigation into what had happened.