The leaders of Britain, France, Germany and the US have issued a joint statement blaming Russia for the nerve agent attack in Salisbury.

The countries said they “abhor the attack” which left ex-Russian spy Sergei Skripal, his daughter and a police officer in a serious condition, adding that it was a clear violation of international law.

It comes after the UK called an urgent UN Security Council meeting to discuss the incident involving a Russian-made nerve agent, as the Nato Secretary General gave his backing to the UK and ahead of an EU Council meeting on the issue next week.

The statement which sees the nations collectively demand answers from Moscow, is boost for Theresa May who on Wednesday expelled 23 Russian diplomats she said were spies, announced asset freezes and new powers to detain people suspected of “hostile activity”.

It joint communique reads: “The United Kingdom briefed thoroughly its allies that it was highly likely that Russia was responsible for the attack. We share the UK assessment that there is no plausible alternative explanation, and note that Russia´s failure to address the legitimate request by the UK government further underlines its responsibility.

“We call on Russia to address all questions related to the attack in Salisbury. Russia should in particular provide full and complete disclosure of the Novichok programme to the Organisation for the Prohibition of Chemical Weapons (OPCW).”

The nations said the use of a “military-grade nerve agent, of a type developed by Russia” was the first offensive use of a nerve agent in Europe since the Second World War, was an “assault on UK sovereignty” and a clear violation of the Chemical Weapons Convention and a breach of international law.

Theresa May: 'We do hold Russia culpable for this brazen, brazen act and despicable act'

Stating that it had threatened the “security of us all”, the statement opened with the words: ”We, the leaders of France, Germany, the United States and the United Kingdom, abhor the attack that took place against Sergei and Yulia Skripal in Salisbury, UK, on 4 March 2018.

“A British police officer who was also exposed in the attack remains seriously ill, and the lives of many innocent British citizens have been threatened. We express our sympathies to them all, and our admiration for the UK police and emergency services for their courageous response.”

The statement went on to raise concerns “against the background of a pattern of earlier irresponsible Russian behaviour”, directly mirroring the language use by Ms May in the House of Commons on Wednesday.

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It follows previous individual supportive statements from each of the countries leaders, but the joint front and specific backing for the UK’s assessment that Russia is responsible adds weight to pressure now being put on Moscow by the international community.

The Russian Tass news agency reported that Vladimir Putin will personally chose the retaliatory measures Moscow takes against the UK following Ms May’s announcements, with diplomatic expulsions likely.

Presidential spokesman Dmitry Peskov said: “A final decision [on retaliatory measures] will, of course, be made by the Russian President. There is no doubt that he will choose the variant that best of all corresponds to the interests of the Russian Federation.”