- Video report by ITV News Political Correspondent Carl Dinnen
Boris Johnson has accused Russia of trying to conceal "the needle of truth in a haystack of lies" over the Salisbury spy poisoning.
Speaking in Brussels, ahead of a meeting with EU counterparts, the Foreign Secretary said Moscow's denials over the incident were "increasingly absurd".
He also accused the Kremlin of changing its story regarding the Novichok nerve agent, the government has said was used in the attack.
Sergei Skripal, a former double agent, and his daughter Yulia, are still fighting for their lives after being exposed to Novichok two weeks ago.
Mr Johnson said: "Today the technical experts from the Organisation for the Prohibition of Chemical Weapons are arriving in the UK to take the samples from Salisbury, and meantime the Russian denials grow increasingly absurd."
He added: "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."
Mr Johnson also said he had been "very heartened" by the strength of support the UK has received since the incident in the Wiltshire city.
The gathering of the EU Foreign Affairs Council has declared its "unqualified solidarity" with the UK.
In a joint statement, the Foreign Affairs Council said: "The European Union takes extremely seriously the UK Government's assessment that it is highly likely that the Russian Federation is responsible.
"The use of chemical weapons by anyone under any circumstances is completely unacceptable and constitutes a security threat to us all.
"The Union calls on Russia to address urgently the questions raised by the UK and the international community and to provide immediate, full and complete disclosure of its Novichok programme to the OPCW.
"The European Union expresses its unqualified solidarity with the UK and its support, including for the UK's efforts to bring those responsible for this crime to justice."
Russia's Tass news agency has reported the Kremlin is insisting the UK either backs up its "unfounded allegations" regarding Moscow's involvement in the Salisbury attack, or apologises.
Kremlin spokesman Dmitry Peskov is quoted saying: "Sooner or later they will have to be responsible for these allegations: they will either have to provide some evidence or apologise."
Its follows Vladimir Putin's dismissal of claims of Russia being behind the Salisbury spy poisoning as "nonsense" as he was re-elected president.
The leader insisted Mr Skripal and his daughter would have died instantly if they had been attacked with a nerve agent.
Mr Putin, who secured a fourth term amid widespread claims of electoral fraud, said he learned about the "tragedy" from the media.
"The first thing that comes to my mind is that should it really be a warfare agent, people would have died instantly," he said.
"It is an obvious fact. Russia does not possess such agents. We have destroyed all our chemical arsenals under control of international observers."
He added: "We are ready for co-operation and said that immediately. We are ready to take part in all necessary probes but the will of the other side is needed for that. So far, we see none."
The national security council will meet in the coming days to discuss Moscow's tit-for-tat response to the UK's expulsion of 23 Russian diplomats.
Mr Johnson is also holding talks with Nato secretary general Jens Stoltenberg on Monday.