Russian Opposition politician Alexei Navalny has been hospitalised and put on ventilator in the intensive care unit, after being allegedly poisoned on board a flight, AP reported on Thursday.

Forty-four-year-old Navalny, an opponent of Russian President Vladimir Putin, felt unwell on a flight back to Moscow from Tomsk, a city in Siberia, his spokesperson Kira Yarmysh said in a tweet. Yarmysh tweeted that Navalny has toxic poisoning. The plane made an emergency landing in Omsk city in Southwestern Siberia.

The hospital said Navalny was in a stable but serious condition, BBC reported. Yarmysh said the doctors told her toxicology reports were delayed. The hospital is now full of police officers, she added.

Yarmysh told a Russian radio station that Navalny was sweating and asked her to talk to him so that he could focus on the sound of her voice. He then went to the washroom and lost consciousness. Yarmsh believed the politician could have consumed poison through the tea he drank on Thursday morning.

Later, she tweeted that “state propaganda” would have people believe that there was no deliberate poisoning, but the fact is that this was no ordinary poison.

But, Anatoly Kalinichenko, the deputy head physician of the hospital, told reporters that it was not certain that Navalny had been poisoned. He said the diagnosis of “natural poisoning” was being considered.

Last year, Navalny, serving jail time, was rushed from prison to hospital, due to suspected poisoning. At the time, doctors said he had a severe allergic attack, and discharged him from hospital the next day. In 2017, Navalny was attacked by several men who threw antiseptic in his eyes, damaging one eye.

Navalny had campaigned to run against Putin in the 2018 presidential elections, but was barred from contesting. He set up a network of campaign offices across Russia and has been putting forth candidates to contest regional elections against Putin’s party, United Russia.

Navalny’s Foundation for Fighting Corruption has been exposing graft among high-level government officials. Navalny has labelled United Russia as the “party of crooks and thieves”. However, last month, Navalny had to shut down the organisation after Yevgeny Prigozin, a businessman with close ties to the Kremlin, filed a lawsuit against him.