Extensive tests have confirmed Alexei Navalny, a fierce critic of President Vladimir Putin, was poisoned.
The Russian opposition leader remains in a coma in a hospital intensive care unit in Germany after he fell ill during a flight back to Moscow.
Berlin’s Charite Hospital, where he is fighting for his life, said on Tuesday (Australian time) that Mr Navalny suffered from “intoxication by a substance from the group of cholinesterase inhibitors”.
That chemical prevents the breakdown of certain neurotransmitters and hampers the ability of nerve cells to communicate with one another in the brain.
The news comes after massive speculation that Mr Navalny was poisoned after drinking a cup of tea that was spiked at an airport cafe before boarding a flight from the Siberian city of Tomsk to Moscow on Thursday.
He was flown to Germany on Saturday for emergency treatment at Charite, Berlin’s leading hospital.
In a statement, the hospital said the 44-year-old was in a serious condition but “there is currently no acute danger to his life”.
It said the prognosis remained uncertain, and long-term consequences, in particular for the nervous system, could not be ruled out at this stage.
The exact substance used to poison Mr Navalny has yet to be identified but what can be ascertained from knowledge of cholinesterase inhibitors is that they are commonly found in insecticides and phosphorus nerve agents.
That’s according to Andrea Sella, professor of inorganic chemistry at University College London, who told CNN that “organic phosphate insecticides’ poisonings are common worldwide – farmers sometimes mishandle them”.
Mr Navalny was initially treated in a hospital in Omsk, Siberia, before doctors there agreed – although not without a lengthy wrangle – to allow him to travel to Germany.
The Russian hospital on Omsk had said there appeared to be no evidence of a malicious poisoning and suggested that Mr Navalny could have fallen into a coma due to low blood sugar levels.
The German hospital’s findings support the claims of Mr Navalny’s team, which has pointed to poisoning at the likely cause from the outset.
Mr Navalny has been the face of domestic opposition to Russian President Vladimir Putin, organising protests against the long-time Russian leader, whom he accuses of perpetuating widespread corruption.
Mr Navalny was in Siberia to support opposition candidates preparing for local elections.
Earlier, the Omsk hospital said it had not been under any political pressure from state authorities.
“We saved his life. There was no outside influence on the care for the patient and there couldn’t have been,” the hospital’s head doctor, Alexander Murakhovsky, said in comments carried by the Russian news agency Interfax.
Dr Murakhovsky said any major decision about the Omsk hospital’s care for Mr Navalny had been decided by a group of up to 10 doctors representing various medical institutions.