British Prime Minister Theresa May said on Monday it was “highly likely” that Moscow was responsible for the attack on a former Russian double agent and his daughter in Salisbury, southern England earlier this month.

Sergei Skripal, 66, and Yulia Skripal, 33, were poisoned by a “military grade” nerve agent, Mrs May said in a statement to the House of Commons.

“This is a group of nerve agents known as Novichok,” she said.

The prime minister chaired a national security council meeting on Monday with senior ministers and security officials to discuss the Skripal attack.

Mrs May said either Russia had ordered the poisoning or it had lost control of the nerve agent used. Moscow must provide an explanation as to what happened by the end of Tuesday, March 13, she said.

Britain would conclude the attack “amounts to an unlawful use of force” if Russia does not give a “credible response” to the ultimatum.

A spokeswoman for the Russian Foreign Ministry dismissed Mrs May’s claim as a “circus show”.

The spokeswoman, Maria Zakharova, said in remarks carried by Russian news agencies that the accusations against Russia represented “another information and political campaign based on a provocation”.

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Earlier on Monday, the Russian embassy in London issued a statement saying the UK government was playing a “very dangerous game” with British public opinion over the Skripal attack.

“We would like to stress once again that we are outraged by the anti-Russian media campaign, condoned by the government, that influences the investigation and has a psychological effect on British residents. Our compatriots and British nationals of Russian origin are worried about their future in this country. UK-based Russian journalists are receiving threats,” the statement read.

“Current policy of the UK government towards Russia is a very dangerous game played with the British public opinion, which not only sends the investigation upon an unhelpful political track but also bears the risk of more serious long-term consequences for our relations.”

American reaction was supportive of the UK without explicitly criticising Russia. Sarah Huckabee Sanders, spokeswoman for US President Donald Trump, 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”.

French President Emmanuel Macron condemned the attack and offered his solidarity with the UK, according to Number 10, during a phone call late on Monday night. The pair discussed wider aspects of aggressive Russian behaviour and agreed to act in concert to counter it.