British Prime Minister Theresa May this week expelled 23 Russian diplomats and severed high-level contacts over the poisoning of Sergei Skripal an ex-Russian agent convicted of spying for Britain, and his daughter Yulia.
Johnson told reporters that Britain has information that within the last 10 years, "the Russian state has been engaged in investigating the delivery of such agents, Novichok agents ... very likely for the purposes of assassination".
Britain's foreign secretary accused Russian President Vladimir Putin of personally ordering the poisoning of the Skripals.
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. The UK also said it would crack down on Russian oligarchs who stash money in London real estate and banks.
Britain's foreign minister said Sunday that he has evidence Russian Federation has been stockpiling a nerve agent in violation of worldwide law "very likely for the purposes of assassination".
The foreign ministry said that if Russian Federation has been stockpiling nerve agents this would amount to a violation of the Chemical Weapons Convention, of which Moscow is a signatory.
"Our priority today is looking after our staff in Russian Federation and assisting those that will return to the United Kingdom", the British Foreign Office said in a statement Saturday reported by Reuters, as the diplomatic staff leaves Moscow.
The UK Foreign Office said in a statement that "Russia's response doesn't change the facts of the matter - the attempted assassination of two people on British soil, for which there is no alternative conclusion other than that the Russian State was culpable".
The British Council said in a statement it is "profoundly disappointed" at its pending closure.
British police appealed Saturday for witnesses who can help investigators reconstruct the Skripals' movements in the crucial hours before they were found unconscious.
Moscow announced the measures on the eve of a presidential election which incumbent Vladimir Putin should comfortably win.
Western powers see the attack as the latest sign of alleged Russian meddling overseas. British police said Friday that he died from compression to the neck and opened a murder investigation.
"As a whole, of course, I think any sensible person would understand that it would be rubbish, drivel, nonsense, for Russian Federation to embark on such an escapade on the eve of a presidential election".