She also told the Echo Moskvy radio station that during the flight Navalny was sweating and asked her to talk to him so that he could “focus on a sound of a voice”. He then went to the bathroom and lost consciousness.

Yarmysh said the politician must have consumed something from tea he drank earlier in the morning at an airport cafe before boarding the plane.

“Doctors are saying the toxin was absorbed quicker with hot liquid,” she tweeted, adding that Navalny’s team called police to the hospital.

Anatoliy Kalinichenko, deputy chief doctor of the Omsk hospital where the politician is being treated, told reporters on Thursday Navalny was in grave yet stable condition. Kalinichenko said doctors are considering a variety of diagnosis, including poisoning, but refused to give details, citing a law preventing doctors from disclosing confidential patient information.

“Doctors aren’t just doing everything possible,” he said. “The doctors are really working now on saving his life.”

“It’s obvious they have done the same thing now,” Yarmysh said.

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For its part, the Kremlin said it had heard reports Navalny was ill and wished him well.

“We are reading this information ... We know that he is in serious condition ... Like with any citizen of the Russian Federation we wish him a speedy recovery,” Kremlin spokesman Dmitry Peskov told journalists, adding that there was no evidence yet of any poisoning.

“As far as we know there are not yet any test results. All assumptions are only assumptions,” he said. “Whether it was a poisoning needs to be confirmed by a laboratory.”

Navalny’s Foundation for Fighting Corruption has been exposing corruption among government officials, including some at the highest level. Last month, he had to shut the foundation after a financially devastating lawsuit from Yevgeny Prigozhin, a businessman with close ties to the Kremlin.

The most prominent member of Russia’s opposition, Navalny campaigned to challenge Putin in the 2018 presidential election, but was barred from running.

He set up a network of campaign offices across Russia and has since been putting forward opposition candidates in regional elections, challenging members of Russia’s ruling party, United Russia. One of his associates in Khabarovsk, a city in Russia’s Far East that has been engulfed in mass protests against the arrest of the region’s governor, was detained just last week after calling for a strike at a rally.

In the interview with Echo Moskvy, Yarmysh said she believed the suspected poisoning was connected to this year’s regional election campaign.

Vyacheslav Gimadi, a lawyer with Navalny’s foundation, said the team is requesting Russia’s Investigative Committee open a criminal probe.

“There is no doubt that Navalny was poisoned because of his political stance and activity,” Gimadi wrote in a tweet on Thursday.

Former US ambassador to Moscow Michael McFaul tweeted: “Praying @navalny will be ok.”

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The European Union’s foreign affairs chief Josep Borrell also expressed concern.

“Worried to hear about Alexei Navalny’s @navalny suspected poisoning,” he wrote on Twitter. “If confirmed, those responsible must be held to account. Wishing him a swift and full recovery.”

The incident involving the highest-profile opposition figure in Russia follows other poisonings of Kremlin critics in and outside the country.

Opposition activist Vladimir Kara-Murza was hospitalised with poisoning symptoms twice – in 2015 and 2017. Both said they believed they were poisoned for their political activity.

Additional reporting by Agence France-Presse, dpa