Doctors are working to save the life of Russian opposition politician Alexei Navalny after he was rushed to intensive care in Siberia suffering from what his spokeswoman said was a suspected poisoning.

Mr Navalny, 44, is unconscious, in intensive care and on an artificial lung ventilator, his spokeswoman, Kira Yarmysh, said on social media.

"We assume that Alexei was poisoned with something mixed into his tea. It was the only thing that he drank in the morning. Doctors say the toxin was absorbed faster through the hot liquid. Alexei is now unconscious," Ms Yarmysh said.

Mr Navalny, a fierce critic of President Vladimir Putin, had started feeling ill when returning to Moscow from Tomsk in Siberia by plane yesterday morning, she added.

He was taken to the hospital in the city of Omsk after he lost consciousness on a flight and his plane made an emergency landing.

"Doctors aren't just doing everything possible. The doctors are really working now on saving his life," the hospital's deputy head doctor Anatoly Kalinichenko told journalists in Omsk. 

Dr Kalinichenko said that no diagnosis had yet been reached and doctors were not sure "as of today that the reason for his condition is poisoning".

He had drunk tea at a cafe at Tomsk airport before boarding his flight. The Interfax news agency quoted the cafe's owners a saying they were checking CCTV cameras to try to establish what had happened.

His plane later made an emergency landing in Omsk so that he could be rushed to hospital, Ms Yarmysh said.

The TASS news agency cited the head doctor at a hospital in Omsk confirming that Mr Navalny had been admitted there and saying he was in a serious condition.

Ms Yarmysh 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," said Ms Yarmysh. She did not say who she believed may have poisoned Mr Navalny, but said police had been called to the hospital.

Alexei Navalny: A constant thorn in the Kremlin's side

Mr Navalny, a lawyer and anti-corruption activist, has served several stints in jail in recent years for organising anti-Kremlin protests.

He has helped release high-impact investigations into what he has said are outrageous examples of official corruption.

A spokesman has said the Kremlin wishes Mr Navalny a "speedy recovery".

Russia holds regional elections next month and Mr Navalny and his allies have been preparing for them, trying to increase support for candidates which they back.

The European Court of Human Rights has ruled that Russia's arrests and detention of Mr Navalny in 2012 and 2014 were politically-motivated and violated his human rights, a ruling Moscow called questionable.

Britain's foreign minister Dominic Raab has said he is "deeply concerned" by the situation, as did the Lithuanian foreign minister, Linas Linkevicius.

"If (poisoning) is confirmed, those responsible must face consequences", Mr Linkevicius said on Twitter.