Boris Johnson hailed the “strength and resolve” of Britain’s allies after holding talks with Nato in Brussels.
It came as scientists from the Organisation for the Prohibition of Chemical Weapons (OPCW) started tests on the substance used to poison Sergei Skripal and his daughter Julia, who remain in a critical condition in hospital.
Mr Johnson said the assassination attempt was “not an isolated case, but the latest in a pattern for reckless behaviour by the Russian state”.
Contrary to Russian allegations, he said the UK was acting in “punctilious accordance” with the Chemical Weapons Convention.
“In the meantime, Russian denials grow increasingly absurd,” Mr Johnson added. “At one time they say that they never made Novichok; at another time they say that they did make Novichok but all the stocks have been destroyed.
“Then again they say that they made Novichok, but all the stocks have been destroyed, but some of them have mysteriously escaped to Sweden or at the Czech Republic or Slovakia or the United States – or even – America, or 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.”
Mr Johnson said that 12 years after the assassination of Alexander Litvinenko in London, “they are not fooling anybody anymore” and that many Nato members had been affected by “malign or disruptive Russian behaviour”.
Britain has already announced the expulsion of 23 Russian spies and is considering other measures, including using new “unexplained wealth orders” against targets.
Mr Putin dismissed claims of Russian involvement in the nerve agent attack as “nonsense“ as he was re-elected President, while his campaign team claimed the diplomatic row “mobilised the nation” in his support.
Tass quoted Kremlin spokesman Dmitry Peskov as saying: “Sooner or later they will have to be responsible for these allegations; they will either have to provide some evidence or apologise.”
After securing a fourth term amid widespread claims of electoral fraud, Mr Putin claimed that Mr Skripal and his daughter would have “died instantly” if a chemical warfare agent was used.
“Russia does not possess such agents,” he added. “We have destroyed all our chemical arsenals under control of international observers … we are ready for cooperation 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.”
But the OPCW has confirmed that neither Russia nor any other state party declared the development of Novichoks – a group of nerve agents roughly translating as “newcomer” that were first developed by the Soviet Union.
Only declared weapons in Russia’s arsenal were destroyed, while the “precursor” chemicals used to make Novichoks can be legally stored for industrial and agricultural uses.
Theresa May insisted that only Russia had the capability, motive and intent to carry out the attack in Salisbury.
“I’m clear that what we have seen shows that there is no other conclusion but that the Russian state is culpable for what happened on the streets of Salisbury,” the Prime Minister said.
Both Nato and the EU Foreign Affairs Council supported the UK’s position, as the diplomatic row continued to escalate on Monday.
Jens Stoltenberg, the Nato Secretary General, said the attack on Mr Skripal and his daughter showed “total disrespect for human lives”.
“The attack was an unacceptable breach of international norms and rules,” he added, offering the alliance’s support to the investigation.
“Russia’s response so far has demonstrated a clear disregard for international peace and security.
“We call on Russia to provide complete disclosure of the Novichok programme to the OPCW.”
The demand was repeated by the EU Foreign Affairs Council, which offered the UK its “unqualified solidarity” amid floundering Brexit talks.
“The lives of many citizens were threatened by this reckless and illegal act,” a statement 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. Any such use is a clear violation of the Chemical Weapons Convention, a breach of international law and undermines the rules-based international order.”
Independent investigators from the OPCW have arrived in the UK and were due to meet with police and officials from the Defence Science and Technology Laboratory at Porton Down, to discuss the process for collecting samples.
They will then be dispatched to international laboratories for tests expected to take a minimum of two weeks, which the British Government hope will verify its analysis under Article 8 of the Chemical Weapons Convention.
The police investigation has continued to widen, with a vehicle used to drive Yulia Skripal to her father’s house after she arrived at Heathrow Airport on 3 March seized for testing.
The pick-up truck, owned by a friend of Mr Skripal, was seen being removed from a cement plant in Durrington by troops wearing protective clothing.
A spokesperson for the Ministry of Defence said operations in the village around 10 miles north of Salisbury was “part of our ongoing support to help police in the investigation”.
There has been speculation that the nerve agent may have been unwittingly carried from Russia by Ms Skripal as she visited her father in Britain.
More than 250 counter-terror detectives are working around the clock on the probe, which Scotland Yard hailed as “one of the largest and most complex investigations” ever undertaken.
Detectives said the process was likely to take “many months”, and it was not possible to say when cordons around areas of Salisbury visited by the victims before they fell ill would be lifted.
“Specialist search officers wearing protective equipment continue to carry out a meticulous, systematic search for evidence to support the investigation,” said a spokesperson for the Metropolitan Police.
“That search is being carried out based on expert scientific advice, to assist detectives in understanding the specific locations that are of most relevance to their lines of enquiry.”
Officers are working through 4,000 hours of CCTV and almost 800 pieces of evidence that have been seized, alongside 400 witness statements.
They continue to appeal for witnesses to come forward, particularly anyone who saw Mr Skripal’s burgundy BMW with registration plate HD09 WAO being driven around on 4 March.
Saturday 3 March: Yulia arrives at Heathrow Airport on a flight from Russia.
Sunday, 4 March: 9.15am: Sergei’s car is seen in the area of London Road, Churchill Way North and Wilton Road.
1.30pm: Sergei’s car is seen being driven down Devizes Road, towards the town centre.
1.40pm: Sergei and Yulia arrive in Sainsbury’s upper level car park at the Maltings. At some time after this, they go to The Mill in the town centre.
2.20pm: They dine at Zizzi restaurant.
3.35pm: They leave Zizzi restaurant.
4.15pm: Emergency services receive a report from a member of the public and police arrive at the scene within minutes, where they find Sergei and Yulia extremely ill on a park bench near the restaurant.
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