LEADERS of the four biggest powers in the Western world have taken the unprecedented step of issuing a joint demand that Russia should fall into line following the Salisbury nerve agent attack.

The US, France and Germany backed Theresa May's assessment and issued a unified statement that they believe Moscow was behind the release of a Russian-made Novichok chemical weapon in the Wiltshire city.

Donald Trump also announced new sanctions to punish Russia for meddling in elections and cyber attacks, heaping pressure on Vladimir Putin and making clear the West is taking any misconduct seriously.

Nato's leading council was on Thursday night set to be briefed on the Salisbury incident, which left ex-Russian spy Sergei Skripal, his daughter and a British police officer all in a serious condition, while across the Channel demands for a joint-EU response gathered momentum ahead of next week's European summit. 

Downing Street said the "quad" of Western nations would now continue to work together to increase pressure on Moscow in a bid to rein in what the nations called "irresponsible Russian behaviour".

The statement was a boost for the Prime Minister, with some doubting whether White House and Élysée Palace administrations that are both trying to build links with Russia would give full-throated support to her position.

But after frenetic diplomatic activity, including telephone calls between the Prime Minister and the other leaders, the statement came saying that Ms May, Mr Trump, French President Emmanuel Macron and German Chancellor Angela Merkel "abhor the attack that took place".

Statements from the quad countries have come before but only at foreign minister level, in July 2017 on Iran's space launch and on Syria's use of chemical weapons at the end of last year.

But the leaders coming together for the first time said in their statement: "The United Kingdom briefed thoroughly its allies that it was highly likely that Russia was responsible for the attack.

This use of a military-grade nerve agent, of a type developed by Russia, constitutes the first offensive use of a nerve agent in Europe since the Second World War. It is an assault on UK sovereignty

"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."

The nations highlighted that the Salisbury attack was the first offensive use of a nerve agent in Europe since the Second World War, adding that it was "an assault on UK sovereignty", a clear violation of the Chemical Weapons Convention and a breach of international law.

Arguing that the incident threatened the security "of us all", the group said: "Our concerns are also heightened against the background of a pattern of earlier irresponsible Russian behaviour."

The US move to impose new unilateral sanctions on Russia served further to underline how the Salisbury attack has become a lynchpin, off which the West is pushing Russia harder over its international activities.

Nineteen people and five Russian organisations were targeted by Washington's measures in what amounts to the most significant action against Moscow since Mr Trump took office.

The sanctions will block those affected from travelling to America, freeze any assets they have in the country and bar US companies from working with them.

In Washington, 40 US Congressmen signed a letter to Ms May pledging "full support" in tackling the Kremlin over Salisbury.

The Prime Minister's security adviser Mark Sedwill was set to brief the North Atlantic Council - Nato's principle political body - on Thursday evening about developments in the Skripal investigation.

Former Nato chief Anders Fogh Rasmussen said ahead of the meeting that "anything short of full solidarity with the UK will be considered a victory by the Kremlin".

Donald Tusk, president of the European Council, will meet with EU leaders next week to discuss the Salisbury incident, but calls for a unified approach are already building.

Belgian Prime Minister Charles Michel called on Mr Tusk to "coordinate a response from the 28 member states of the European Union".

The leaders of Canada and Australia have already offered their backing for the UK's position on Salisbury.

London was nonetheless braced for the Russian response to Ms May's dramatic announcement on Wednesday that she would expel 23 diplomats from the country's UK embassy, who she claimed were part of an "espionage network".

Ms May, who personally visited Salisbury on Thursday, also used a statement in the Commons to announce asset freezes and new powers to detain people suspected of "hostile activity".

The Russian Tass news agency reported that Vladimir Putin will personally choose the retaliatory measures Moscow takes against the UK following Ms May's announcements, with tit-for-tat 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."

He said that Russia is "perplexed and does not comprehend the British leadership's stance" and insisted the "accusations are unsubstantiated".

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