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

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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 on Monday to investigate the samples used in the attack and the results should be known in about two weeks, Britain's foreign ministry said.

Britain said the stockpiling of nerve agents would amount to a violation of the Chemical Weapons Convention, which Moscow has signed.

Russia's ambassador to the European Union, Vladimir Chizhov, told the BBC his country had 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 and said Russia's reaction "was not the response of a country that really believes itself to be innocent".

"Their response has been a mix of smug sarcasm and denial and obfuscation," he said.

Skripal and his daughter may have been exposed to the nerve agent used in their attempted assassination through his car ventilation system, intelligence sources told the US television channel ABC news.

The sources said the toxin was used in a "dust-like powdered form" and that it circulated through the vents of the car, the channel reported.

Johnson said Britain's National Security Council would meet later in the week to decide "what further measures, if any" may be taken, and that the government could decide to target Russian wealth in Britain.

The British capital has been dubbed 'Londongrad' due to the large quantities of Russian money that have poured in since the collapse of the Soviet Union in 1991.

Some British lawmakers have urged Prime Minister Theresa May to freeze the private assets of members of Russian President Vladimir Putin's circle.

Reuters