Britain's foreign Minister Dominic Raab said he had summoned Russia's ambassador to the UK to express his concern at the poisoning of Alexei Navalny.
Navalny, a prominent critic of President Vladimir Putin, was airlifted to Germany last month after collapsing on a flight from the Siberian city of Tomsk to Moscow. Germany has concluded he was poisoned with Novichok.
"Today the UK summoned Russia's Ambassador to the UK to register deep concern about the poisoning of Alexey @navalny," Raab said on Twitter.
"It's completely unacceptable that a banned chemical weapon has been used and Russia must hold a full, transparent investigation."
Moscow says it has seen no evidence he was poisoned.
Raab added that he was relieved to hear Navalny had been taken out of his medically-induced coma.
In a separate statement, a foreign office spokesperson said: "There is a case here for Russia to answer. This took place on Russian soil, against a Russian citizen. They have international obligations to uphold.
"This is nothing short of an attack against the rules based international system which keeps our societies safe."