Investigators believe a nerve agent was used to attack a former Russian spy and his daughter who were found slumped on a public bench in southern England.
The Guardian quotes sources close to the investigation, saying they would not discuss exactly which nerve agents were used, but the best known are VX and sarin.
Counter-terrorism police are investigating the case, which has left Sergei Skripal, 66, and his 33-year-old daughter, Yulia, in critical condition.
Amid speculation about who is behind the suspected poisoning, Moscow said the case was being used to fuel an "anti-Russian campaign" and further strain ties with Britain.
Britain's Home Secretary, the minister responsible for public safety, called a meeting today NZT of the government's emergency committee, where senior officials will consider their response.
While police say they are keeping an open mind about the case, it has reminded Britain of the 2006 poisoning of former spy Alexander Litvinenko, which was blamed on the Russian state.
Foreign Secretary Boris Johnson told lawmakers yesterday that the government would act - possibly downgrading England's participation in this year's World Cup in Russia - if Moscow is shown to have been involved in the Skripal case.
Johnson warned British officials may not take part in the sport event "in the normal way," but did not elaborate.
The Russian Foreign Ministry said the allegations were intended to "further exacerbate relations between our countries."
"What happened to Skripal has been immediately used to further incite anti-Russian campaign in Western media," ministry spokeswoman Maria Zakharova said.
Skripal, a former colonel in Russia's GRU military intelligence service, was convicted in 2006 of spying for Britain and imprisoned.
He was freed in 2010 as part of a widely publicized spy swap in which the US agreed to hand over 10 members of a Russian sleeper cell found operating in America in return for four Russians convicted of spying for the West.