Alexei Navalny, the most prominent political opponent to Russia’s president Vladimir Putin, was taken to hospital on Thursday after his flight was diverted en route to Moscow. His spokeswoman said that Mr Navalny had fallen ill on the aircraft after drinking tea at the airport before departure.

The 44 year-old started perspiring and fell unconscious, Kira Yarmish said. She said: “This morning Navalny was returning to Moscow from Tomsk. During the flight, he felt ill. The plane made an urgent landing in Omsk. Alexei has toxic poisoning.”

She added: “We suspect that Alexei was poisoned by something mixed into [his] tea. It was the only thing he drank since morning. Doctors are saying that the toxic agent absorbed faster through the hot liquid. Right now Alexei is unconscious.”

The BBC has quoted a fellow passenger as saying: “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.”

Although there were reports that Mr Navalny had regained consciousness, at 08:58 GMT, (14:58 local time), Ms Yarmish said that he was still unconscious and in intensive care.

The deputy head doctor at Omsk Emergency Hospital said that doctors were trying to save his life.

Following a previous poisoning, while he was in prison for organising unauthorised demonstrations, Navalny wrote on Facebook:

“Are they really such idiots to poison you in a place where suspicions point only at them? It’s a good question. For now, I can say one thing with certainty: the people in power in Russia are really quite stupid guys. It seems to you that in their actions you need to look for secret meaning or a rational purpose. But in fact, they are just stupid, malicious and obsessed with money.”

The apparent attempt on Navalny’s life comes 80 years to the day after the assassination of Leon Trotsky on the orders of Stalin.

PHOTO: Video footage on social media shows Mr Navalny being stretchered to an ambulance on the airport runway