Foreign Secretary Dominic Raab said he is “deeply concerned” about the suspected poisoning of a prominent opponent of Russian President Vladimir Putin.
Russian opposition activist Alexei Navalny is in a coma and on a ventilator in a hospital intensive care unit, with allies claiming the suspected poisoning is linked to his political activity.
The 44-year-old felt unwell on a flight back to Moscow from Tomsk, a city in Siberia.
Mr Navalny’s spokeswoman Kira Yarmysh suggested he may have consumed something in a cup of tea he drank at an airport cafe before boarding the plane.
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.
Mr Raab said “I am deeply concerned” by reports that Mr Navalny “has been poisoned on a flight to Moscow and is now in a coma in intensive care”.
“My thoughts are with him and his family,” the Foreign Secretary added.
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.
Mr Navalny is one of the most prominent opposition figures in Russia and campaigned to challenge Mr Putin in the 2018 presidential election, but was barred from running.
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.