British Foreign Secretary Boris Johnson said yesterday that Russia has been stockpiling the deadly nerve agent used to poison a Russian former double agent in England and has been investigating how such weapons can be used in assassinations.

Britain has said Russia used the Soviet-era nerve agent called Novichok to attack Sergei Skripal and his daughter Yulia in the first known offensive use of such a weapon on European soil since World War Two. Russia has denied any involvement.

"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," Johnson told the BBC.

Britain and Russia have each expelled 23 diplomats over the attack as relations between the two countries crash to a post-Cold War low.

Skripal, a former colonel in Russian military intelligence who betrayed dozens of Russian agents to Britain, and his daughter are fighting for their lives after they were found collapsed on a bench in the city of Salisbury two weeks ago.

Officials from the world's chemical weapons watchdog will arrive in Britain today to investigate the samples used in the attack and the results should be known in about two weeks, Britain's foreign ministry said.

Russia's ambassador to the EU told the same programme that his country has destroyed its reserves of such substances and a British research laboratory could be the source of the nerve agent used in the attack. Johnson dismissed those claims.