Boris Johnson has described the poisoning of Russian dissident Alexey Navalny as “shocking” and called for a “transparent investigation” into events.
Mr Navalny, who has been an outspoken critic of Russian president Vladimir Putin, fell ill on a flight in Siberia last week after he is thought to have been poisoned.
He is now being treated in Germany.
Speaking on Wednesday, the prime minister said the poisoning of the Russian opposition leader was “the most extraordinary thing”.
“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,' Mr Johnson said.
“We now have the most extraordinary thing, which is perhaps the leading domestic opponent of Vladimir Putin being poisoned in Germany.
“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.”
Earlier in the day, Mr Johnson took to Twitter to call for a "transparent investigation" and branded the poisoning "shocking".
“The poisoning of Alexey Navalny shocked the world. The UK stands in solidarity with him & his family. We need a full, transparent investigation into what happened," Mr Johnson wrote.
“The perpetrators must be held accountable & the UK will join international efforts to ensure justice is done.”
Mr Navalny was airlifted from a hospital in Omsk, Russia, to Berlin amid concerns he may not receive the necessary care needed.
Russian doctors said there was no evidence to suggest Mr Navalny had been poisoned, but German authorities said their tests had shown otherwise.
German Chancellor Angela Merkel said Russia must fully investigate Mr Navlany’s poisoning and hold its perpetrators to account.
The 44-year-old fell ill on a flight back to Moscow from the Siberian city of Tomsk on Thursday and was taken to hospital after the plane made an emergency landing in Omsk.
His supporters believe he was poisoned and that the Kremlin is behind it.
Mr Navalny’s wife told reporters Russian hospital staff and men she suspected were law enforcement agents did not allow her speak to the German specialists, who she said were brought into the facility in secrecy, through a back door.
“I was forcibly kicked out in a rude manner,” Yulia Navalnaya said, her voice shaking.
“This is an appalling situation. They are not letting us take Alexei. We believe that clearly something is being hidden from us.”