The German government has said traces of the nerve agent Novichok have been found in tests on samples taken from the Russian opposition leader Alexei Navalny, who is a patient in a hospital in Berlin.

Navalny, a strong critic of the Russian president, Vladimir Putin, fell ill on a flight back to Moscow from Siberia on 20 August.

He was taken to a hospital in Omsk and later transferred to Berlin, where doctors said there were indications he had been poisoned.

Steffen Seibert, spokesman for the German chancellor, Angela Merkel, said in a statement that testing by a special military laboratory had shown proof of a chemical nerve agent from the Novichok group.

Novichok, a Soviet-era nerve agent, was used to poison the former Russian spy Sergei Skripal and his daughter in Britain. It is a cholinesterase inhibitor, part of the class of substances that doctors at the Berlin hospital the Charité initially identified in Navalny.

Seibert said the German government would inform its partners in the European Union and Nato about the test results. He said it would consult with its partners in light of the Russian response on an appropriate joint response.

Navalny’s allies in Russia have insisted he was deliberately poisoned by the country’s authorities, accusations that the Kremlin rejected as empty noise.

The Russian doctors who treated Navalny in Siberia have repeatedly contested the German hospital’s conclusion, saying they had ruled out poisoning as a diagnosis and that their tests for poisonous substances came back negative.

This a breaking news story. More details to come …