Theresa May has given Vladimir Putin 24 hours to explain how a deadly Russian nerve agent was used in an assassination attempt on British soil, as she said it is “highly likely” Moscow was responsible.

She vowed Britain will pursue “extensive measures” unless a credible account can be given as to how military-grade Novichok was deployed, leaving an ex-spy, his daughter and a British policeman in hospital.

Retaliatory action that might be announced as early as Wednesday could potentially include a larger military presence in Eastern Europe, fresh sanctions and action at the United Nations.

Government sources told The Independent British ministers have also discussed expelling the Russian Ambassador in response to what Ms May branded a “reckless and despicable act”. Ms May said if there is no “credible response” by the end of Tuesday, then the UK would conclude there has been an “unlawful use of force” by Moscow.

The White House immediately backed Britain on Monday, branding the attack as “reckless, indiscriminate and irresponsible”, but Moscow dismissed the Prime Minister’s statement as a “circus”.

Ms May revealed experts at Britain’s Porton Down defence laboratory have confirmed the Salisbury attack involved highly-specialised Novichok, first developed by the Soviet Union in the 1970s.

Updating the Commons, she explained that as a result it is “highly likely that Russia was responsible” for the act against ex-spy Sergei Skripal and his daughter Yulia, which also left Wiltshire Police Detective Sergeant Nick Bailey in a serious condition and parts of the medieval cathedral city off limits.

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The Prime Minister went on: “This attempted murder using a weapons-grade nerve agent in a British town was not just a crime against the Skripals.

“It was an indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk.

This attempted murder using a weapons-grade nerve agent in a British town was not just a crime against the Skripals. It was an indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk. Prime Minister Theresa May

“And we will not tolerate such a brazen attempt to murder innocent civilians on our soil.”

She argued there are only two plausible explanations for the attack on 4 March – a “direct act by the Russian state against our country” or that Russia allowed the nerve agent to fall “into the hands of others”.

MPs were told that Foreign Secretary Boris Johnson called-in Russia’s Ambassador Alexander Yakovenko to demand answers, with sources later telling The Independent the meeting just before 4pm was only five minutes long.

Describing the tone of the face-to-face, a Foreign Office insider said it was “cool but firm, and there was no handshake”.

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The “outrage” felt by the British public was made clear to Mr Yakovenko, who was told his country had until midnight on Tuesday to explain itself.

Downing Street officials explained that cabinet ministers and security chiefs will discuss the Russian response at a meeting of the National Security Council on Wednesday, ahead of a potential further statement to the Commons that afternoon.

Speaking to MPs on Monday, Ms May set out a “well-established pattern of Russian state aggression” – including invading Crimea, fomenting overseas conflict, violating European airspace and meddling in other country’s elections.

She then went on to give a strong indication of potential avenues for action, saying the UK is committed to collective defence through NATO “in the face of Russian behaviour”.

She added: “Indeed our armed forces have a leading role in Nato’s Enhanced Forward Presence with British troops leading a multinational battle group in Estonia.

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

“We have led the way in securing tough sanctions against the Russian economy. And we have at all stages worked closely with our allies and we will continue to do so.

“We must now stand ready to take much more extensive measures.”

She also signalled a push at the UN saying that the attack had been an “unlawful use of force”, something that would constitute a breach of Article 2 (4) of the UN Charter.

One minister told The Independent: “We have already been discussing kicking the ambassador out, it had seemed too early before today. But things are different now.”

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Commons Foreign Affairs Committee chairman and Conservative MP Tom Tugendhat said Russia’s actions were “deeply threatening to the British people”.

While a string of Labour MPs stood to show solidarity with Ms May’s approach, their leader Jeremy Corbyn chose to make a political point about donations the Conservatives have received from Russian business people.

Mr Putin dismissed claims about his state’s alleged involvement on Monday, telling a BBC reporter who questioned him on a visit to a grain centre: “We’re dealing with agriculture here ... and you talk to me about some tragedies.

“Get to the bottom of things there, then we’ll discuss this.”

The Kremlin has denied involvement, while the Russian embassy accused Britain of playing a “very dangerous game” and warned of “serious long-term consequences” and Russian foreign ministry spokeswoman Maria Zakharova branded Ms May’ statement “a circus show in the British parliament”.

Andrei Lugovoi, one of the two men accused of assassinating Alexander Litvinenko with radioactive polonium in 2006, said Britain’s response to events was suspiciously quick. Evidence, he said, that London was operating according to its own script.

Alexander Litvinenko, killed after being poisoned in 2006, and Andrei Lugovoi (right) accused of the murders, who said the UK was not seeking the truth in the Skripal case (Getty Images)

“Any chemist or physicist will tell you that as a minimum you need some kind of serious expertise on a serious expert level to determine whether or not a country is responsible,” he told the Interfax news agency. “When such statements are made in the course of a few days, this speaks only of irresponsibility and the fact that they haven’t set out to discover the truth.”

The White House response did not directly mention Russia, but said: “The use of a highly lethal nerve agent against UK citizens on UK soil is an outrage.

“The attack was reckless, indiscriminate and irresponsible…we stand by our closest ally in the special relationship that we have.”