It is “highly likely” that Russia was behind the attack on former Russian double agent Sergei Skripal, who was attacked by a “military grade” nerve agent a week ago in Salisbury, the UK prime minister Theresa May said in a speech on March 12.

She identified the nerve agent as from a group of nerve agents known as Novichok that were developed by the Soviet Union during the Cold War and as such is fairly conclusive proof that the attack originated in Russia.

However, rather than just blame the Russian government directly for the assassination attempt on the ex-spy, May was careful to offer the caveat Russia may have lost control of its chemical weapons stock.

"Either this was a direct action 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,” said May in a speech to the House of Commons.

The Russian ambassador to London has been called in and the British government are demanding a "full and complete disclosure" of the Novichok programme to the international body the Organisation for the Prohibition of Chemical Weapons.

She gave Moscow until the end of March 13 to respond and will give another speech to the house on March 14 where she says she will lay out UK response to the attack.

Britain’s allies rallied round on March 12 promising support. US Secretary of State Rex Tillerson said the US agreed with the UK that Russia was likely to be behind the attack. "We agree that those responsible – both those who committed the crime and those who ordered it – must face appropriately serious consequences," he said in remarks reported by the BBC.

May also said that the attack was being treated as an individual incident and not as an act of war. She added it was not a “article five matter,” referring to the clause in the Nato treaty that says an attack on one nation is considered an attack on all the members of Nato.

The Russians were dismissive. Russian Foreign Ministry spokeswoman Maria Zakharova said May's statement was "a circus show in the British parliament".