The PM pledges to join “international efforts to ensure justice is done” and offers his “solidarity” to Mr Navalny and his family.
Boris Johnson has called for a “transparent” investigation into the poisoning of Russian opposition politician Alexei Navalny, which he described as “the most extraordinary thing”.
The 44-year-old – a renowned critic of Vladimir Putin – fell ill and collapsed last Thursday on an internal Russian flight from Tomsk, in Siberia, to Moscow after drinking what his supporters suspect was poisoned tea.
Mr Navalny is being treated with atropine, which is used to treat certain types of nerve agent and pesticide poisonings, as well as some types of slow heart rate and to decrease saliva production during surgery.
The German doctors have said they cannot currently determine whether Mr Navalny will have lasting issues, “especially in the area of the nervous system”.null
He remains in a serious condition at the Charite hospital “but there is currently no acute danger to his life”, the doctors added.
In a tweet on Wednesday, Mr Johnson said: “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. The perpetrators must be held accountable & the UK will join international efforts to ensure justice is done.”
On a visit to a school in Leicestershire later on Wednesday, the prime minister said: “We have had poisonings of people who have fallen out with the Russian regime and who have left Russia.
“We saw what happened with Alexander Litvinenko, what happened to Sergei and Yulia Skripal in our country.”
Mr Johnson – wrongly referring to the location of Mr Navalny’s suspected poisoning as Germany rather than Russia – added: “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.
“Those who are responsible must be held to account and must be brought to justice.”
Mr Johnson’s comments came after German Chancellor Angela Merkel called on Russian police to “investigate this crime in detail and in full transparency”.
Mr Navalny is known as the Russian president Mr Putin’s most prominent opponent.
Through his Anti-Corruption Foundation, Mr Navalny has sought to expose fraud and illicit activity by members of Mr Putin’s United Russia party, which he famously called “the party of crooks and thieves”.
In turn, Russian authorities have levied money-laundering allegations against him, imprisoned him and raided the homes and offices of his associates.
In 2017, Mr Navalny was attacked by a pro-Kremlin supporter who threw a chemical substance in his eye, leaving him partially-blind.
Last year, he was rushed to hospital from his prison cell, again for suspected poisoning.