Video report by ITV News Security Editor Rohit Kachroo
Boris Johnson has condemned the poisoning of Russian opposition leader Alexi Navalny with the nerve agent Novichok.
A German military lab confirmed Mr Navalny was poisoned with the Soviet-era chemical weapon earlier on Wednesday, a move Chancellor Angela Merkel called "attempted murder".
Mr Navalny, a politician and corruption investigator who is one of Russian President Vladimir Putin’s fiercest critics, fell ill on a flight back to Moscow from Siberia on August 20 and was taken to hospital in the Siberian city of Omsk after the plane made an emergency landing.
He was later transferred to Berlin’s Charite hospital, where doctors last week said there were indications he had been poisoned.
Rohit Kachroo discusses the international response to the poisoning
He is still in a serious condition and the hospital said he faces a long road to recovery.
The Prime Minister said: "It’s outrageous that a chemical weapon was used against Alexey Navalny.
"We have seen first-hand the deadly consequences of Novichok in the UK.
"The Russian government must now explain what happened to Mr Navalny – we will work with international partners to ensure justice is done."
Novichok was used to poison former Russian spy Sergei Skripal and his daughter, Yulia, in Salisbury in March 2018, sparking a major investigation by British authorities and international condemnation.
Responding to the announcement about Mr Navalny by the German authorities, his spokesperson, Kira Yarmysh, told ITV news: "I am sure that only Putin could have made the decision to poison Navalny.
"All the more so now, given that it has been proven that it was Novichok, which is at the disposal of the Russian security services."
After the finding was announced on Wednesday, Ms Merkel said: “With this, it is certain that Alexei Navalny is the victim of a crime.
"He was meant to be silenced and I condemn this in the strongest possible manner.”She added: “There are very serious questions now that only the Russian government can answer, and must answer.”
Ms Merkel described what happened to Navalny as ”the attempted murder by poisoning of one of Russia's leading opposition figures".
Leaders from across the world have been quick to condemn the use of the nerve agent on Mr Navalny.
UK Foreign Secretary Dominic 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."
Nato secretary-general Jens Stoltenberg has also condemned the suspected Novichok attack on Alexei Navalny as "truly shocking".
In a statement, he said: “The German government has announced that Alexei Navalny was the victim of an attack with a chemical nerve agent from the Novichok group.
"The use of a military-grade nerve agent makes it even more urgent that the Russian authorities conduct a full and transparent investigation.
He added: “Nato regards any use of chemical weapons as a threat to international peace and security.”
President of the European Council Charles Michel said he "condemned the move in the strongest possible terms."
French Foreign Minster Jean-Yves Le Drian condemned the use of the nerve agent and said it was Russia's responsibility to answer the "serious questions raised" by the issue.
Earlier on Wednesday, Chancellor Angela Merkel’s spokesperson, Steffen Seibert said the results from the military lab were “proof without doubt of a chemical nerve agent from the Novichok group.”
Both Skripals eventually recovered from the poisoning but Dawn Sturgess, a woman who later came in to contact with it died.
Theresa May, who was Prime Minister at the time of the Skripals were poisoned said on Twitter: "Deeply alarming that Novichok has again been used in an attempt to silence critics of the Russian state.""The use of chemical weapons is illegal and barbaric."
The nerve agent is a cholinesterase inhibitor, part of the class of substances that doctors at the Charite initially identified in Mr Navalny.
Mr Seibert said the German government will inform its partners in the European Union and Nato about the test results, and will consult with its partners in light of the Russian response “on an appropriate joint response”.
He added: “The German government condemns this attack in the strongest terms.”
Jerry Smith, who works for CHC Global explains to ITV News how the Novichok poisoning could be used to send a message
Mr Navalny’s allies in Russia have insisted he was deliberately poisoned by the country’s authorities, accusations the Kremlin rejected as “empty noise”.
The Russian doctors who treated him in Siberia have repeatedly contested the German hospital’s conclusion, saying they had ruled out poisoning.