Angela Merkel said Alexei Navalny, the Russian opposition activist, had been poisoned with the chemical nerve agent novichok in an attempt to murder him, and demanded an explanation from the Kremlin.
The German chancellor said she was “appalled” at the attack, and condemned it “in the strongest terms”.
“It is now certain that Alexei Navalny was the victim of a crime — he was supposed to be silenced,” Ms Merkel told reporters in Berlin.
The German government said a toxicology test by a specialist military laboratory on samples taken from Mr Navalny had proved “beyond all doubt” that he had been poisoned with a “chemical nerve agent of the novichok group”.
Novichok, a military-grade nerve agent developed by the Soviet Union, was also used in the 2018 Salisbury poisonings that targeted former Russian intelligence officer Sergei Skripal and his daughter Yulia.
“There are serious questions which only the Russian government can answer and must answer,” said Ms Merkel.
The “crime” against Mr Navalny “was directed against all the basic values and the fundamental rights that we espouse”, she added.
“The fate of Alexei Navalny has attracted international attention, and the world is waiting for answers.”
Dmitry Peskov, spokesman of Russian president Vladimir Putin, told the Interfax news agency that Moscow was “ready and interested in total co-operation and exchange of information on the subject” with Berlin.
He said Russia could not “react in a qualified manner” to accusations Mr Navalny was poisoned with novichok until Berlin provided Moscow with evidence confirming the accusations.
“Before the patient was flown to Berlin, a whole series of analyses was conducted in Russia in accordance with all international standards and no toxic substances were discovered,” Mr Peskov added.
German-Russian relations were already strained by the murder of a Chechen rebel in a Berlin park last year and a massive hack on the Bundestag computer system in 2015. The poisoning of Mr Navalny threatens to plunge them to a new low.
Some experts now expect Germany to impose a fresh round of sanctions against Moscow. In its statement, the government said it would consult with its partners about an “appropriate joint response” to the poisoning.
Amid growing alarm in Moscow at the prospect of western countermeasures, the rouble weakened by 2 per cent on the German government statement to trade at 75 to the dollar. Russia’s RTS futures index fell nearly 3 per cent after having risen throughout Wednesday.
One western intelligence officer said the use of novichok in the poisoning of Mr Navalny was a strong indicator of state-backed involvement. “Given the history of this nerve agent and how it’s been used, it would be highly surprising if this hadn’t been sanctioned at an extremely senior level,” the officer told the Financial Times.
On Wednesday, British foreign secretary Dominic Raab said he was “deeply concerned” by the German finding and threatened “consequences” for using chemical weapons prohibited by international treaties.
“The Russian government has a clear case to answer,” Mr Raab said in a statement. “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.”
The British authorities blamed the Salisbury attack on the Skripals on the GRU, Russia’s military intelligence service, and the UK and its western allies responded by expelling dozens of Russian diplomats.
The White House said it was “deeply troubled” by the news about the poisoning, which was “completely reprehensible”.
“Russia has used the chemical nerve agent novichok in the past. We will work with allies and the international community to hold those in Russia accountable, wherever the evidence leads, and restrict funds for their malign activities,” said John Ullyot, spokesperson for the White House National Security Council.
Marco Rubio, the Florida Republican who heads the Senate intelligence committee, said the revelation confirmed that there had been “yet another attempted assassination of a Putin opponent”.
French foreign minister Jean-Yves Le Drian condemned the use of novichok against Mr Navalny as “shocking and irresponsible” and said France was in touch with Germany and other partners to co-ordinate a response, particularly through the Organisation for the Prohibition of Chemical Weapons (OPCW).
“Once again, the ban on the use of any chemical weapon has been violated,” he said in a statement. “I want to recall that the use of chemical weapons in any place and at any time, by anyone in any circumstances, is unacceptable and contravenes international norms against the use of such weapons.”
Heiko Maas, foreign minister, said Russia’s ambassador to Germany was summoned to the foreign ministry on Wednesday and told that the German government insisted the Russian authorities “fully and transparently investigate” Mr Navalny’s poisoning.
“Russia should itself have a serious interest in good relations with its neighbours in Europe,” Mr Maas said. “It must now be clear that this is the right point to make a contribution to that.”
Mr Navalny, Russia’s most prominent anti-corruption activist and the best-known critic of Mr Putin, fell sick on a flight from Siberia after drinking a cup of tea at the airport in Tomsk on August 20.
He was initially treated in the nearby city of Omsk, where his plane made an emergency stopover. Doctors there said they found no traces of poison in his system, diagnosed him with a metabolic disorder and acute pancreatitis, and refused to let him travel for medical treatment abroad.
Days later Mr Navalny was flown to the Charité hospital in Berlin where he has been treated ever since. Charité said on Wednesday Mr Navalny was “continuing to improve” but that his recovery was “likely to be lengthy”.
“It is still too early to gauge the long-term effects which may arise in relation to this severe poisoning,” it said, adding that Mr Navalny was being treated in an intensive care unit and remains on a ventilator.
Charité announced last week that tests indicated he had been poisoned with a cholinesterase inhibitor and was being treated with the antidote atropine. Experts said that indicated the activist had been exposed to a nerve agent such as novichok, sarin or VX.
The suspected attack on Mr Navalny has highlighted the risks Kremlin critics and human rights campaigners face in Russia. It is the sixth apparent poisoning of a Russian opposition activist in the past five years, with two previous victims reporting symptoms similar to Mr Navalny’s.
Russia’s foreign ministry spokeswoman Maria Zakharova accused Germany of deliberately ignoring the Kremlin’s offers to help investigate Mr Navalny’s poisoning.
“Instead of a thorough investigation, painstaking collaboration meant to discover the real answer, our partners prefer yet more public statements without any backing at all,” Ms Zakharova said, calling the claims “yet another information campaign” against Russia.
Additional reporting by Demetri Sevastopulo in Washington and Helen Warrell