Video report by ITV News Correspondent John Ray
A Russian dissident has said opposition politician Alexei Navalny has been 'poisoned' in similar circumstances to an experience of his own.
44-year-old Navalny, a critic of Russia's President Vladimir Putin, felt unwell on a flight back to Moscow from Siberia, his spokeswoman, Kira Yarmysh, said.
She tweeted: “The plane made an emergency landing in Omsk. Alexei has a toxic poisoning.”
Mr Navalny is unconscious and was placed on a ventilator in an intensive care unit.
According to Ms Yarmysh, he may have consumed something in a cup of tea he drank at an airport cafe before boarding the plane.
Pyotr Verzilov, a friend of Navalny, fellow Russian dissident, and occasional manager of band Pussy Riot, said: "The symptoms do definitely follow what happened to me two months ago in September 2018.
"I was also poisoned in...under controlled circumstances".
Pyotr Verzilov suggests that his friend Navalny was poisoned
During the flight, Mr Navalny started sweating and asked her to talk to him so he could "focus on a sound of a voice". He then went to the toilet and lost consciousness.
“Doctors are saying the toxin was absorbed quicker with hot liquid,” she said, adding that Mr Navalny’s team called police to the hospital.
Footage posted on Twitter shows Navalny being transported into an ambulance by emergency workers.
Britain's Foreign Secretary Dominic Raab said he is "deeply concerned" by reports of a suspected poisoning.
Mr Raab said: "My thoughts are with him and his family."
Commons Foreign Affairs Committee chairman Tom Tugendhat suggested the Kremlin was responsible.
"Another opponent of Putin's mafia state is poisoned," he said.
"Alexei Navalny is in hospital as the Kremlin again demonstrates the brutality of the Putin regime."
But Russian state news agency Tass reported that police were not considering deliberate poisoning, citing an anonymous source who said: "It is not unlikely that he drank or consumed something yesterday himself."
Anatoliy Kalinichenko, deputy chief doctor of the hospital in Omsk, Siberia, where Mr Navalny is being treated, said he was in grave but stable condition.
Dr Kalinichenko said medics are considering a variety of diagnoses, including poisoning.
Like several other opposition voices in Russia, Navalny has been detained by law enforcement and harassed by pro-Kremlin groups on numerous occasions.
Last year, he was rushed to hospital from prison with what his team said was suspected poisoning. Doctors subsequently said he had had a severe allergic attack and discharged him back to prison the following day.
Mr Navalny's Foundation for Fighting Corruption has focused on exposing sleaze among government officials, but last month he had to shut the organisation after a financially devastating lawsuit from Yevgeny Prigozhin, a businessman with close ties to the Kremlin.
Mr Navalny has also been accused by Belarus's authoritarian leader Alexander Lukashenko of organising the mass protests against his rule in recent days following an election which has been widely criticised as rigged.
Pyotr Verzilov says he believes the Kremlin is worried about the Belarus uprising
Vyacheslav Gimadi, a lawyer with Navalny’s foundation, said the team has called for Russia’s Investigative Committee to open a criminal probe.
“There is no doubt that Navalny was poisoned because of his political stance and activity,” Gimadi tweeted on Thursday.