International experts are to test the nerve toxin used on a former Russian double agent in the city of Salisbury.
Yulia and Sergei Skripal. Photo: Facebook/ AFP
Sergei Skripal and his daughter Yulia are still in a critical condition, two weeks after the attack.
British weapons experts say the nerve agent is of Russian origin, and the Kremlin has been asked to explain how it came to be used in England - if Russia itself did not authorise the attack.
The Kremlin has denied involvement, and Russia's ambassador to Europe has now suggested the nerve toxin could have come from Britain's own military research facility at Porton Down, noting it is not far from Salisbury.
British foreign secretary Boris Johnson says chemical weapons specialists from the Organisation for the Prohibition of Chemical Weapons will arrive later today.
The results are expected to take a "minimum of two weeks".
He dismissed a suggestion by Russia's EU ambassador Vladimir Chizhov that the agent might have come from the Porton Down lab in Wiltshire.
Skripal poisoning: Chemical weapons experts arrive in UK
Mr Johnson told the BBC's Andrew Marr Show: "We actually have evidence within the last 10 years that Russia has not only been investigating the delivery of nerve agents for the purposes of assassination, but has also been creating and stockpiling Novichok."
He said Russia's reaction to the incident "was not the response of a country that really believes itself to be innocent".
Prime Minister Theresa May has said Russia is "culpable" for the attack.
Unconfirmed reports from the US suggest that the nerve agent used may have been introduced into the ventilation system of a car belonging to Mr Skripal.
- BBC