Boris Johnson has demanded Russia explain itself after the "outrageous" Novichok poisoning of prominent Kremlin critic Alexei Navalny.
The Prime Minister pledged to “work with international partners to ensure justice is done”.
The German government said toxicology tests at a military laboratory showed "unequivocal proof" that the military-grade nerve agent was deployed.
Mr Navalny, a thorn in the side of Russian President Vladimir Putin, collapsed on a plane on August 20 and was taken to a hospital in the Siberian city of Omsk after the plane made an emergency landing.
The anti-corruption activist was transferred to Berlin’s Charite hospital, where he remains in a coma, after Russian doctors repeatedly ruled out poisoning.
The Kremlin has denied any involvement. It comes as Foreign Secretary Dominic Raab called on Russia to “tell the truth” about the incident, saying he was “deeply concerned”.
Novichok was used by suspected Russian spies to target former agent Sergei Skripal and his daughter Yulia in Salisbury, Wiltshire, in March 2018.
Mr Raab said: “It is absolutely unacceptable that this banned chemical weapon has been used again, and once more we see violence directed against a leading Russian opposition figure.
“The Russian government has a clear case to answer. It must tell the truth about what happened to Mr Navalny.
“We will work closely with Germany, our allies and international partners to demonstrate that there are consequences for using banned chemical weapons anywhere in the world.”
Chancellor Angela Merkel’s spokesman Steffen Seibert said in a statement that testing by a special German military laboratory had now shown “proof without doubt of a chemical nerve agent from the Novichok group”.
“It is a dismaying event that Alexei Navalny was the victim of an attack with a chemical nerve agent in Russia,” Mr Seibert said.
“The German government condemns this attack in the strongest terms.”
Ms Merkel said Mr Navalny was the victim of an “attempted murder by poisoning” and the aim was to silence him.
She said there are “very serious questions that only the Russian government can answer and must answer”.
Germany’s foreign minister Heiko Maas said the Russian ambassador has been summoned and told that Berlin expects a full and transparent investigation.
The revelation by Germany risks further souring relations between Moscow and the West. The White House branded the apparent poisoning "completely reprehensible" and vowed to hold Russia accountable.
Allies of Mr Navalny, who heads one of Russia’s most prolific opposition movements - Russia of the Future - say he was poisoned on orders of Mr Putin, a claim the Kremlin denies.
Last week, Russia's Prosecutor General's Office insisted a preliminary inquiry had found no evidence "deliberate criminal acts committed against" Mr Navalny.
Novichok is a cholinesterase inhibitor, part of the class of substances that doctors at the Charite initially identified in Mr Navalny and a military-grade nerve agent developed by the Soviet Union at the end of the Cold War.
Russian opposition figures have a history of falling victim to assassination attempts by suspected Kremlin agents, some of which have been successful, such as in the case of Alexander Litvinenko who drank tea laced with Pulonium-210 in a London hotel in 2006.
Following the Salisbury poisonings, Britain charged two Russians – alleged to be agents of the Russian military intelligence service GRU – in absentia with the 2018 attack, which left the Skripals in a critical condition and killed a local woman. Russia has refused to extradite the men to the UK.