Britain gives Moscow two days to explain alleged use of nerve agent from Russia (VIDEO)

She alleges the attack was either a direct act by the Russian state on Britain, or the Russian government allowed its nerve agent ‘Novichok’ to get into the wrong hands. “The government has concluded that it is highly likely that Russia was responsible,” she said.

“Either this was a direct act by the Russian State against our country, or the Russian government lost control of this potentially catastrophically damaging nerve agent and allowed it to get into the hands of others.”

She added that Boris Johnson, the foreign secretary, summoned the Russian ambassador to the foreign office on Monday. He said Russia must explain which of the two possible explanations is the correct one. She says the ambassador must reply by the end of Tuesday.

May says if Russia does not give a “credible response” the government will conclude that the attack involved “unlawful use of force by the Russian state against the United Kingdom.” May says if the government does come to that conclusion, she will return to the Commons to outline retaliatory proposals.

“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,” she added.

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

Skripal, 66, and his 33-year-old daughter have been in hospital in a critical condition since March 4 when they were found unconscious on a bench outside a shopping center in Salisbury.

May says the government will be considering whether dignitaries and ministers from the UK will be attending the World Cup. But she did not say anything about the England team boycotting the event, which suggests that that is not on the agenda.

Labour leader Jeremy Corbyn has called for a “robust” dialogue with Russia. “We need to continue seeking a robust dialogue with Russia on all the issues dividing our countries, both domestic and international – rather than simply cutting off contact and simply letting tensions and divisions get worse, and potentially even more dangerous.”

He caused uproar among Tory MPs by raising the issue of £800,000 donated by Russian oligarchs to the Conservative Party.

The Prime Minister chaired a National Security Council (NSC) meeting on Monday, which brought together senior ministers with intelligence and security officials.

Russia has denied any involvement in the attack.