Boris Johnson has demanded a “transparent” probe into the suspected poisoning of Russian opposition politician Alexei Navalny.
The Prime Minister said the incident was “the most extraordinary thing” and demanded that those responsible be brought to justice.
His intervention comes amid a deterioration in relations with Moscow following tensions over the novichok nerve agent attack in Salisbury in 2018.
Mr Navalny, a leading critic of Russian president Vladimir Putin, was taken ill on an internal flight from Tomsk in Siberia to Moscow on August 20.
He has since been transferred to a German hospital, where he remains in a coma.
Mr Johnson tweeted: “The poisoning of Alexey Navalny shocked the world. The UK stands in solidarity with him and his family.
“We need a full, transparent investigation into what happened.
“The perpetrators must be held accountable and the UK will join international efforts to ensure justice is done.”
On a visit to a school in Leicestershire, Mr Johnson expressed alarm at the incident.
He said: “We have had poisonings of people who have fallen out with the Russian regime and who have left Russia, we are used to – we saw what happened with Alexander Litvinenko, what happened to Sergei and Yulia Skripal in our country.
“We now have the most extraordinary thing, which is perhaps the leading domestic opponent of Vladimir Putin being poisoned in Germany.”
Although Mr Navalny is being treated in Germany, he fell ill during a flight in Russia.
Mr Johnson went on: “You really have to ask yourself what has gone on here and I think what we want to see is a full, thorough, transparent investigation into the poisoning of Alexei Navalny and those who are responsible must be held to account and must be brought to justice.”
The Kremlin has rejected accusations of involvement in the alleged attack on Mr Navalny, whose allies claim he was poisoned.
The latest row comes after explosive claims from Westminster’s Intelligence and Security Committee of Russian meddling in British politics.
Russian hackers were accusing of breaking into former Cabinet Minister Liam Fox’s email account to steal secret documents on UK-US trade talks.
The dossier came to prominence during the 2019 election campaign when Jeremy Corbyn claimed the papers proved ministers were discussing the NHS with US trade negotiators.
Earlier this month, the Royal Navy escorted nine Russian warships away from British waters after they sailed from the Baltic Sea into the North Sea.
