A government source told the Times that security forces "knew pretty much by the time of the first Cobra [the emergency co-ordination briefing that took place the same week] that it was overwhelmingly likely to come from Russia."
Security sources told the Times they believe Russians conducted tests to see whether novichok could be used for assassinations.
The international chemical weapons watchdog on Wednesday rejected Russia's call for a joint investigation into the Salisbury poisoning. But Russia said the number of countries that abstained from the vote suggested many have doubts about Britain's allegations that Moscow was behind the attack and now plans to take its denials of involvement to the U.N. Security Council.
Britain said Russia's proposal for a joint investigation received only six votes at a special session of the executive council of the Organization for the Prohibition of Chemical Weapons. The council has representatives from 41 countries.
Russia requested the session in The Hague, Netherlands, to push its repeated rejection of Britain's claim that Moscow orchestrated the poisonings of Sergei Skripal and his daughter, Yulia. The British government has invited experts from OPCW to help identify the substance that sickened the Skripals on March 4.
"The purpose of Russia's ludicrous proposal at The Hague was clear - to undermine the independent, impartial work of the international chemical weapons watchdog," British Foreign Secretary Boris Johnson said in a statement.
Russia's OPCW envoy Alexander Shulgin said the 17 abstentions from Wednesday's vote meant "more than half of the members of the executive council refused to support the U.S. and U.K. position."
The United States and more than two dozen British allies have expelled more than 150 Russian diplomats in a show of solidarity over the attack. Moscow has responded in kind.
Russia's U.N. ambassador called for an open meeting Thursday of the U.N. Security Council about the case. Ambassador Vassily Nebenzia made the request at the end of his speech Wednesday to a council meeting on chemical weapons in Syria.