Aides of Russian opposition figure Alexei Navalny said Thursday they discovered traces of a Novichok nerve agent on a bottle taken from the hotel where he stayed before falling ill in a suspected poisoning attack.

The 44-year-old lawyer and outspoken critic of President Vladimir Putin collapsed last month on a flight from the Siberian city of Tomsk to Moscow after a campaign trip to support opposition candidates in local elections.

His team wrote on Instagram that a German laboratory had found traces of a “military-grade toxin” collected from the hotel room in Tomsk where the opposition leader and his team stayed.

The aides said they believed Navalny was poisoned “before he left his room to reach the airport”.

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Navalny is being treated in a hospital in Berlin and on Tuesday said he was breathing for the first time without medical support.

Germany has said it has “unequivocal evidence” that Navalny was poisoned with a Novichok nerve agent and this week reported that labs in France and Sweden had confirmed the findings.

The post said that Navalny’s team collected the bottle and other items that may have left toxin traces in the hotel room after hearing he fell ill.

“It was decided to take everything that could be hypothetically useful and hand it over to doctors in Germany,” the post said.

“The fact that the case would not be investigated in Russia was also quite obvious.”

The anti-corruption campaigner’s suspected poisoning has sparked sharp condemnation from Western leaders, who have called for a thorough investigation and for those responsible to be brought to justice.

Russia has dismissed “unsubstantiated claims” over the incident and said its doctors found no trace of toxins.