Boris Johnson has said the UK is in possession of evidence that Russia has been exploring nerve-agent based assassinations and that the country has been stockpiling deadly chemical weapons in the last decade.

The Foreign Secretary in particular claimed Britain has reason to believe Moscow has been collecting the “military grade” Novichok nerve agent that the UK Government says was deployed in the Salisbury attack.

He made the comments moments after a senior Russian diplomat claimed his country has no stockpile of any nerve agent, and even suggested the source of the chemical used in Salisbury was the UK’s Porton Down military laboratory.

It comes as the war of words escalated over the 4 March incident, which left ex-Russian spy Sergei Skripal, his daughter and a British policeman in serious conditions, with UK insiders promising further economic sanctions against Vladimir Putin and his allies.

Speaking to BBC’s Andrew Marr show, Mr Johnson said: “We actually have evidence within the last ten years that Russia has not only been investigating the delivery of nerve agents for the purposes of assassination, but has also been creating, stockpiling, Novichok.”

With international chemical weapons experts coming to the UK to begin independent testing of samples from Salisbury, the Foreign Office indicated this line of investigation could form part of a probe into what it claimed is a breach of the Chemical Weapons Convention.

But Russia’s ambassador to the EU Vladimir Chizhov had denied moments earlier on the same programme, that his country had any nerve agents in its military arsenal, saying there are, “no stockpiles whatsoever”.

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He also excluded the possibility of any chemical weapons “fleeing Russia after the collapse of the USSR, but added that there are “specialists” who left his country and travelled to the UK.

Mr Chizhov then went on to suggest the nerve agent used in Salisbury may have come from the Porton Down laboratory, which is not far from the English cathedral city – though he admitted he had absolutely no evidence for it.

He told Marr: “When you have a nerve agent or whatever, you check it against certain samples that you retain in your laboratories.

“Porton Down, as we now all know, is the largest military facility in the United Kingdom that has been dealing with chemical weapons research. It’s actually only eight miles from Salisbury.”

Asked directly whether Porton Down was “responsible” for the attack, Mr Chizhov said: “I don’t know. I don’t have any evidence of anything having been used.”

The Foreign Office out out a statement warning against Russian attempts to ”muddy the waters” over Salisbury, meanwhile Mr Johnson confirmed experts from the Organisation for the Prohibition of Chemical Weapons will arrive in the UK on Monday to begin testing samples of the nerve agent identified by British scientists.

Vladimir Putin's Photo ops – Russia's Man of Steel? Show all 20 1 /20 Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin is pictured with a horse during his vacation outside the town of Kyzyl in Southern Siberia on August 3, 2009. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin aims at a whale with an arbalest to take a piece of its skin for analysis on the Olga Bay, some 240 kilometres north-east of Nakhodka on August 25, 2010. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin plunges into the icy waters of lake Seliger during the celebration of the Epiphany holiday in Russia's Tver region AFP/Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin (top) takes part in a judo training session at the "Moscow" sports complex in St. Petersburg, on December 22, 2010. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin works out at a gym at the Bocharov Ruchei state residence in Sochi on August 30, 2015. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin fishes in the remote Tuva region in southern Siberia. The picture taken between August 1 and 3, 2017. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin (L) and the leader of the Night Wolves biker group, Alexander Zaldostanov (R), also known as the Surgeon, ride motorcycles on August 29, 2011 at a bikers' festival in the Black Sea port of Novorossiysk, Russia. Putin described leather-clad bikers as brothers and boasted of the "indivisible Russian nation" after roaring into a biking rally on a Harley Davidson. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin wears glasses as he visits the Technology Park of the Novosibirsk Academic Town in Novosibirsk on February 17, 2012. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin rides a horse during his vacation outside the town of Kyzyl in Southern Siberia on August 3, 2009. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin holds a pistol during his visit to a newly-built headquarters of the Russian General Staff's Main Intelligence Department (GRU) in Moscow, 08 November 2006. ?Some countries are seeking to untie their hands in order to take weapons to outer space, including nuclear weapons,? Putin said at the Chief Military Intelligence Department on Wednesday. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin sits inside a T-90AM tank during a visit to an arms exhibition in the Urals town of Nizhny Tagil on September 9, 2011 Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin wears a helmet and the uniform of the Renault Formula One team before driving a F1 race car on a special track in Leningrad region outside St. Petersburg on November 7, 2010. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin guides a boat during his vacation in the remote Tuva region in southern Siberia. The picture taken between August 1 and 3, 2017. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin hunts fish underwater in the remote Tuva region in southern Siberia. The picture taken between August 1 and 3, 2017. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin is seen at the Russian boxing team training club after casting his vote for the Russian Presidential election, 14 March 2004 in Moscow. Putin coasted to a landslide victory with 69.0 percent of the vote in Sunday's election, according to the first exit poll aired on Russian television moments after voting ended across the country's 11 time zones. AFP/Getty Images Vladimir Putin's Photo ops – Russia's Man of Steel? Russian President Vladimir Putin poses for a picture inside the Tupolev-160 strategic bomber jet at the Moscow's Chkalovsky military airport, 16 August 2005. President Vladimir Putin took off from Moscow for a supersonic flight in a cruise-missile carrying Tupolev-160 bomber jet, the latest in the Russian leader's action-packed public appearances. After a health check, Putin donned a flight suit and took the commander's position in the strategic bomber, which was piloted by Major General Anatoly Zhikharev, with a colonel and a lieutenant colonel in charge of navigation, Russian media reported. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? A picture released on March 6, 2010 shows Vladimir Putin look through binoculars in the Karatash area, near the town of Abakan, during his working trip to Khakassia, on February 25, 2010. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin measuring a polar bear on the island Alexandra Land, part of the Franz Josef Land archipalego in the Arctic Ocean. Putin, better known in the West for his tough-guy image, expressed concern for the fate of Arctic polar bears threatened by climate change. "The polar bear is under threat. Their population is currently only 25,000 individuals," Putin was quoted by Russian news agencies as saying Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Vladimir Putin carries a hunting rifle during his trip in Ubsunur Hollow Biosphere Reserve in Tuva Republic in this undated picture released on October 30, 2010 by RIA Novosti news agency. Getty Vladimir Putin's Photo ops – Russia's Man of Steel? Russian President Vladimir Putin pilots a motorized hang glider while flying with cranes as he takes part in a scientific experiment as part of the "Flight of Hope", which aims to preserve a rare species of - cranes on September 5, 2012. 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The team from The Hague will meet with officials from the Defence Science and Technology Laboratory at Porton Down and the police to discuss the process for collecting samples, including “environmental” issues.

Samples will then be dispatched to “highly reputable” international laboratories selected by the OPCW for testing with results expected to take a minimum of two weeks.

A statement from the Foreign Office repeated Mr Johnson’s claims about Russia stockpiling Novichok in the last decade, adding: “The start of the investigation by the international chemical weapons watchdog comes as the Foreign Secretary travels to Brussels to brief foreign ministers from across the European Union on the attempted assassinations in Salisbury before meeting with the NATO Secretary General.