THERESA May has given Moscow until the end of the night to explain why a “military grade nerve agent developed by Russia” had been used in the UK against the spy Sergei Skripal and his daughter Yulia.

In a tense statement to the House of Commons yesterday afternoon, the Prime Minister said it was “highly likely” that the government of Vladimir Putin was behind the attempted murder in Salisbury last week.

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The substance used has been identified as Novichok, which translates as “newcomer” in Russian, and was part of a group of advanced nerve agents developed by the Soviet Union in the 1970s and 1980s.

The Russians described May’s statement as a “circus show”.

May told MPs: “Based on the positive identification of this chemical agent by world-leading experts at Porton Down, our knowledge that Russia has previously produced this agent and would still be capable of doing so, Russia’s record of conducting state-sponsored assassinations, and our assessment that Russia views some defectors as legitimate targets for assassinations, the Government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal,” she said.

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The Prime Minister said there were now just two explanations: “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.”

Boris Johnson has summoned the Russian Ambassador, who will tonight be expected to say which of those two explanations is right.

May told MPs: “Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom.”

According to AP, a spokeswoman for the Russian foreign ministry described the British accusation as a “circus show”.

The ministry 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”.

First Minister Nicola Sturgeon called for “cool heads” but said there needed to be a “firm response”.

“Russia simply cannot be allowed to launch attacks on our streets with impunity,” she tweeted.

Her party colleague Ian Blackford also had strong words for Moscow.

He told MPs: “There can be no denying that this assassination attempt on Mr Skripal and his daughter is not only a step too far – but calls into questions every aspect of our current and future relationship with Russia.

“This ruthless action from Russia put not only the lives of our emergency services at risk but also threatened the safety of the wider public.

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“There must be firm and strong action taken to send a clear message to the Kremlin: we will not accept Russian interference in our democracy or way of life.

“This kind of international outrage must never be seen again.”

Labour leader Jeremy Corbyn faced shouts of “shame” and “disgrace” from Tory MPs as he criticised May’s party for taking money off Russian oligarchs.

He told the Commons: “We’re all familiar with the way huge fortunes, often acquired in the most dubious circumstances in Russia, sometimes connected with criminal elements, have ended up sheltering in London and trying to buy political influence in British party politics.

“Meddling in elections, as the Prime Minister put it, and there has been over £800,000 worth of donations to the Conservative Party from Russian oligarchs and their associates. If that is the evidence before the Government, even before the investigation in Salisbury is complete, the Government could be taking action to introduce new financial sanctions powers.”

Skripal and his daughter are still in a critical condition in hospital. The spy was a colonel in Russian military intelligence who was recruited by MI6 as an informant.

He was captured and sentenced to 13 years in prison in 2006 but served four years before being swapped as part of a deal to deport 10 Russian spies working in the US.

Sergeant Nick Bailey, who was contaminated after responding to the emergency remains in hospital in a serious condition, but has been able to talk to family.