Britain has accused Russia of breaching international law by secretly manufacturing and stockpiling the deadly nerve agent used in the Salisbury attack over the past decade, despite having claimed to have destroyed its arsenal.

Foreign Secretary Boris Johnson said he had information demonstrating that Russia has not only continued to accrue chemical weapons, but is exploring how they can be used for targeted assassinations.

British officials are likely to share evidence with international chemical weapons experts who arrive in the UK on Monday to assess samples from the Salisbury attack, which the UK says involved a Russian-made Novichok nerve agent.

If the Kremlin is found culpable for transporting the nerve agent to Salisbury, or to have lied about previously destroying chemical stockpiles, the experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) can impose penalties.

It comes as Vladimir Putin is on course to retain the presidency until 2024, following Russian elections characterised by rising anti-West sentiment.

Mr Johnson indicated on Sunday that the UK has intelligence showing Russia lied when in declared to the OPCW last year that it had destroyed all of its 40,000 metric tons of chemical weapons.

He said: “We actually have evidence within the last 10 years that Russia has not only been investigating the delivery of nerve agents for the purposes of assassination, but has also been creating and stockpiling Novichok.”

Boris Johnson: We have evidence that Russia has been creating and stockpiling Novichok

The team of OPCW investigators from The Hague will meet experts from Porton Down’s Defence Science and Technology Laboratory and the police to discuss the process for collecting samples linked to the Salisbury attack, which left former spy Sergei Skripal, his daughter and a British policeman in serious conditions.

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

We actually have evidence within the last 10 years that Russia has not only been investigating the delivery of nerve agents for the purposes of assassination, but has also been creating and stockpiling Novichok Foreign Secretary Boris Johnson

The Foreign Office said Ms May’s invite to the OPCW reflects the UK’s commitment to comply with obligations under the Chemical Weapons Convention (CWC), which demands the independent assessment of scientific work.

But one well-placed government source told The Independent it was also about highlighting how Russia was violating the convention.

The source said: “The approach from the start has been to build the case in international law to maximise the possibility of multilateral action, and the strength of action that might be taken.

Theresa May warns Russia UK will 'never tolerate threat on life of British citizens'

“If you want people in the EU or UN to stand up to Russia, you have to give them the ground on which to do it.

“That’s why she has focussed on Salisbury being an ‘unlawful use of force’, which speaks to the UN charter, and [the CWC] is another element to increase pressure.”

The convention is an international treaty that bans chemical weapons, requires their destruction within a specified period of time and is overseen by the OPCW.

Both the UK and Russia are signatories, with Moscow having signed the convention in 1993 and declared all its stockpiles of chemical weapons destroyed under OPCW supervision in 2017.

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. At the helm of a motorized hang glider that the birds have taken as their leader, Putin made three flights - the first to get familiar with the process, and two others with the birds. AFP/Getty

If states are found by the OPCW to have engaged in “prohibited actions”, such as lying about what stockpiles they have, the organisation can find it in breach and impose penalties, including bars on trading certain chemical products.

The OPCW can also bring the issue before the UN Security Council and General Assembly, where the UK has already begun a drive to isolate Moscow.

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

In claims branded “nonsense” by British officials, he went on to suggest the nerve agent used in Salisbury may have come from the UK’s own Porton Down laboratory.

Salisbury spy attack: Russian official suggests nerve agent could have come from UK lab Porton Down

He said: “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 by the BBC 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.”

On Sunday Ms May had a call with the Polish Prime Minister, Mateusz Morawiecki, with the pair agreeing on “a determined response from the European Union” towards Russia.

Mr Johnson travels to Brussels on Monday to brief foreign ministers from across the bloc on the attempted assassinations in Salisbury before meeting with Nato Secretary General Jens Stoltenberg.

The European Council is set to meet at the end of this week and is expected to offer further international support to Theresa May’s push to hold Russia to account.