MOSCOW • Russian opposition politician Alexei Navalny was in a coma in a Siberian hospital yesterday after drinking a cup of tea that his spokesman said she believed was laced with poison.

Mr Navalny, a fierce critic of Russian President Vladimir Putin, started feeling ill when returning to Moscow from Tomsk in Siberia by plane on Wednesday morning. He was carried off the plane on a stretcher after it made an emergency landing at Omsk.

He was in intensive care and on an artificial lung ventilator in an Omsk hospital, his spokesman Kira Yarmysh said.

"We assume that Alexei was poisoned with something mixed into his tea. It was the only thing that he drank in the morning. Alexei is now unconscious," she said.

Doctors gave contradictory information about his condition, saying that it had stabilised, but also that there was still a threat to his life and they were working to save him.

There is a long history of Kremlin foes being poisoned or falling ill after suspected poisonings.

They include Mr Alexander Litvinenko, who died in London in 2006 after drinking tea laced with polonium-210, and Mr Sergei Skripal, a former double agent who was poisoned with a nerve agent in 2018 in Salisbury, England.

The Kremlin has repeatedly denied involvement in those and other incidents, calling them anti-Russian provocations.

Ms Yarmysh said the police had been called to the hospital. Regional health ministry spokesman Tatyana Shakirova confirmed that Mr Navalny has been hospitalised in Omsk and said doctors had assessed his condition as serious.

"Doctors are doing everything possible to stabilise his condition," she said. "The poisoning version is one of several versions being considered. It's not possible now to say what the reason was."

Mr Navalny, 44, a lawyer and anti-corruption activist, has served many stints in jail for organising anti-Kremlin protests and has been physically attacked in the street by pro-government activists. He has also helped investigations into what he has said are outrageous examples of official corruption.

Russia holds regional elections next month and Mr Navalny and his allies have been working to increase support for the candidates they back.

Passenger Pavel Lebedev, who was on the same plane as Mr Navalny, posted an account of what he saw on social media.

"At the start of the flight he went to the toilet and didn't come back. He started feeling really sick. They struggled to bring him round and he was screaming in pain."

Footage posted on social media showed a motionless Mr Navalny being stretchered into an ambulance by medics.

S7, the airline he was travelling with, said he had not eaten or drunk anything on board.

Mr Navalny had had a cup of tea at a Tomsk airport cafe before the flight, Ms Yarmysh said.

She drew a parallel with an incident last year in which Mr Navalny suffered an acute allergic reaction that one doctor said could have resulted from poisoning with an unknown chemical.

"Obviously the same has been done to him now," she added.

The Kremlin yesterday said it wished Mr Navalny well.

"We know that he is in serious condition... Like any citizen of the Russian Federation we wish him a speedy recovery," said Kremlin spokesman Dmitry Peskov.

REUTERS, AGENCE FRANCE-PRESSE