Both Sergei Skripal and his daughter remain in a critical condition after being exposed to a deadly poison developed in the former Soviet Union.
Arriving at a meeting with EU counterparts in Brussels, Foreign Secretary Boris Johnson slammed the Kremlin ahead of investigators arriving to examine the nerve agent used in the attack.
He said: “The Russian denials grow increasingly absurd.
“At one time they say that they never made Novichok, and at another time they say they did make Novichok, but all the stocks have been destroyed … but some of them have mysteriously escaped to Sweden, or the Czech Republic, or Slovakia, or the United States, or even … the United Kingdom.
“I think what people can see is that this is a classic Russian strategy of trying to conceal the needle of truth in a haystack of lies and obfuscation.”
“The Russian denials grow increasingly absurd. “At one time they say that they never made Novichok, and at another time they say they did make Novichok, but all the stocks have been destroyed … ”
Mr Johnson pointed to the assassination of Alexander Litvinenko in London as evidence of their previous activity.
He added: “There is scarcely a country round the table here in Brussels that has not been affected in recent years by some kind of malign or disruptive Russian behaviour.
“And that is why, I think, the strength and the resolve of our European friends is so striking today.”
Weapons experts are visiting the UK today to examine the nerve agent used to poison the former Russian spy.
They remain in hospital in a critical condition.
Vladimir Putin dismissed claims of Russian involvement in the Salisbury spy poisoning as “nonsense” as he was re-elected president of the Federation.
And a Kremlin spokesman called Britain’s allegations “slanderous, groundless, and difficult to explain”.
He added that the UK will soon have to provide evidence of Russia’s involvement in the case, or apologise.