The British government has concluded it is “highly likely” that Russia was responsible for the attempted murder of a former Russian intelligence officer and his daughter last week in the English city of Salisbury, Prime Minster Theresa May said on Monday.

While the police investigation into the attack on Sergei Skripal and his daughter Yulia is not yet complete, May told Parliament that chemical weapons experts from a nearby British military facility reported that the two were “poisoned with a military-grade nerve agent of a type developed by Russia,” one of a group known as Novichok.

“There are, therefore, only two plausible explanations,” the prime minister said, “either this was a direct act by the Russian state against our country, or the Russian government lost control of its potentially catastrophically damaging nerve agent and allowed it to get into the hands of others.”

May informed the House of Commons that the British foreign secretary, Boris Johnson, had already summoned the Russian ambassador in London to demand an explanation from his government by the end of Tuesday.

The prime minister also said that Britain would “respond to this reckless and despicable act,” noting that previous incidents had been met with sanctions and the expulsion of diplomats.

The Russian Foreign Ministry spokesperson, Maria Zakharova, quickly dismissed May’s statement as “a circus act” that was part of a “political campaign based on provocation.”

Jeremy Corbyn, the leader of the opposition Labour party, argued that effective measures might include targeting the property and investments in Britain of Russian nationals with government ties. After noting a recent report that May’s Conservative party had received donations of more than 820,000 pounds (over $1.1 million) from Russian nationals since she became prime minister in 2016, Corbyn called on the government to support an amendment proposed by Labour to the United Kingdom’s sanctions and money-laundering bill aimed at making life in Britain more difficult for Russian officials and oligarchs.

Johnson had hinted last week that retaliation for the attack, if it was sponsored by Russia, could interfere with England’s participation in the World Cup, which begins in Russia in June.

Alexei Navalny, the leading opposition figure in Russia, suggested that Britain should sanction three figures who have amassed fortunes through their connections to Russian President Vladimir Putin.

The list includes Roman Abramovich, the billionaire owner of Chelsea Football Club in London; Alisher Usmanov, an oligarch who owns a large stake in Arsenal Football Club; and Igor Shuvalov, Russia’s deputy prime minister, who owns an apartment in central London worth nearly $17 million according to Navalny’s anti-corruption foundation.

Another form of possible retaliation proposed by some members of parliament was removing the British broadcast license for the local version of Russia Today, or RT, the Potemkin news network directly financed by the Kremlin. That suggestion struck several close observers of Russia as a terrible idea. Max Seddon, the Moscow correspondent of the Financial Times, and Alexey Kovalev, a Russian journalist who exposes propaganda and lies in the Russian state media, argued that the network has little impact but shuttering it would give Putin’s government an excuse to interfere with the more important work done by British reporters in Russia.

Russian officials and state media outlets have spent the past week arguing, without evidence, that the attack might have been carried out by the British intelligence service, as part of a plot to tarnish Russia’s reputation ahead of the World Cup soccer tournament it is hosting this summer.

“Before inventing any more fairy tales,” Zakharova added, the British government should explain who was responsible for the murder of another former spy, Alexander Litvinenko, and “many others who mysteriously died on British soil.”

A British public inquiry into the murder of Litvinenko reported in 2016 that he had been poisoned in London in 2006 by two Russian agents who spiked his tea with the radioactive isotope polonium-210. The murder, the inquiry’s report concluded, had most likely been carried out by Russia’s Federal Security Service, or FSB, and was “probably approved by” Putin.

“The Soviet Union invented Novichok agents and pioneered their manufacturing techniques, which are still very much shrouded in secrecy,” Dan Kaszeta, a chemical weapons expert based in London, explained in a series of tweets. “They were specifically developed to evade the West/NATO’s detection capabilities and foil intelligence collection efforts,” he explained. “The USSR then Russia went to great lengths to keep the programme secret at a point when the USSR had already agreed in principle to chemical arms control. Their existence is a political embarrassment.”

“At least one of these Novichok agents is a solid at normal temperatures,” Kaszeta added. “It could be deployed as a dust or powder. This could explain a slower absorption as the absorption rate could be controlled by particle size.”

The Skripals remain in serious condition, along with a British police officer who came to their aid after they collapsed on a park bench in Salisbury on March 4.

Sergei Skripal, a former colonel for Russia’s military intelligence service, has lived in Britain since 2010. Four years before that, he had been convicted by a military court in Moscow of acting as a double agent for the British intelligence service MI6 and sentenced to 13 years in prison. He was pardoned and released in 2010, along with three other men, as part of an exchange for 10 Russians caught spying on the United States that year.

A still image from CCTV footage recorded on February 27, 2018 shows former Russian spy Sergei Skripal buying groceries at the Bargain Stop convenience store in Salisbury on February 27, 2018.<br /><br /><br />
British detectives on March 8 scrambled to find the source of the nerve agent used in the "brazen and reckless" attempted murder of a Russian former double-agent and his daughter. Sergei Skripal, 66, who moved to Britain in a 2010 spy swap, is unconscious in a critical but stable condition in hospital along with his daughter Yulia after they collapsed on a bench outside a shopping centre on Sunday.<br /><br /><br />
 / AFP PHOTO / -        (Photo credit should read -/AFP/Getty Images)

The former double agent, Sergei Skripal, was recorded on a security camera last month while buying groceries in Salisbury, England.

Photo: AFP/Getty Images

Russian television’s most important anchor, Dmitry Kiselyov, devoted part of his news broadcast Sunday night to advancing the conspiracy theory that the attack on the Skripals was a false flag

“Only the British stand to benefit,” the man once chosen by Putin to explain Russia to the world said, before speculating that the British government, driven by a form of anti-Russian racism, Russophobia, was trying to invent an excuse to organize an “international boycott of the World Cup.”

“Why not poison him?” the anchor said, according to a translation from Guardian correspondent Andrew Roth. “Is he so valuable? And do it with his daughter to turn it into a real tear-jerker for the public.”

The theory that Britain is out to spoil the World Cup, which has been promoted widely across Russian state media, hinges on the fact that England tried and failed to secure the right to host this year’s tournament. As Jane Mayer explained in a New Yorker profile of Christopher Steele, the former British intelligence agent hired to find an explanation for Donald Trump’s strange affinity for Russia, “Steele’s first client after leaving MI6 was England’s Football Association, which hoped to host the World Cup in 2018, but suspected dirty dealings by the governing body, FIFA. England lost out in its bid to Russia, and Steele determined that the Kremlin had rigged the process with bribes.”

The Russian embassy in London, meanwhile, has used its Twitter feed to mock reporting on the British investigation, even complaining that Skripal, a former double agent for British intelligence released by Moscow in a spy swap in 2010, would be better described as a British rather than a Russian spy.

As the writer Oliver Bullough observed in The Guardian on Sunday, it might be unwise for Britain to wait around for the United States to agree to any coordinated effort against Russia in retaliation for the attack. “Donald Trump can be counted on to troll Sadiq Khan whenever there’s a terrorist attack in London,” Bullough reminded readers, “but he is yet to bother tweeting about Sergei Skripal, his daughter, Yulia, DS Nick Bailey or the 18 other people affected by the nerve agent used last Sunday.”

Yaroslav Trofimov, a Wall Street Journal columnist, suggested that the incident could test how committed the Trump administration is to its relationship with Britain.

Asked to comment on the British government’s findings in the White House press room on Monday, Sarah Sanders, Trump’s spokesperson, denounced the attack but refused to comment on the attribution of responsibility to Russia.

Update: March 13, 4:28 am EST (8:28 am GMT) This report was updated to add more information about the nerve agent British experts say was used in the attack in Salisbury, and note the calls to ban the British version of Russia Today.