Boris Johnson, the British foreign secretary, released a statement on the organization’s report, saying, “There can be no doubt what was used and there remains no alternative explanation about who was responsible — only Russia has the means, motive and record.”

Mr. Skripal, 66, is a former Russian military intelligence officer who has lived for several years in Salisbury; his daughter, 33, lives in Russia and was visiting him at the time of the poisoning.

“The attack on Sergei and Yulia Skripal in Salisbury was the first time a nerve agent had been deployed in Europe since the Second World War,” Mr. Fleming said.

On Wednesday night, the Metropolitan Police released a statement attributed to Ms. Skripal, turning down a Russian offer of consular assistance, adding another irritant to worsening relations with Moscow. Russia has repeatedly requested access to Ms. Skripal, accusing Britain of improperly denying it contact with one of its citizens.

“I have been made aware of my specific contacts at the Russian Embassy, who have kindly offered me their assistance in any way they can,” Ms. Skripal was quoted as saying in the statement. “At the moment, I do not wish to avail myself of their services, but if I change my mind, I know how to contact them.”

Russian officials have ridiculed British accusations, while floating an array of theories, some of them contradictory, about what happened in Salisbury. They claim that neither the Soviet Union nor Russia ever had a novichok program, that other former Soviet bloc countries could have supplied the novichok, that the toxin involved was not novichok, and that the British might have poisoned the Skripals.

The statement attributed to Ms. Skripal, 33, met with derision from the Russian authorities, who described it as “an interesting read,” pointedly noting that there was no way to verify it, and suggesting that the remarks contained more questions than answers.