THERESA May is bracing herself today for Russian retaliation after she received a major diplomatic boost when America, France and Germany joined forces with Britain to blame Moscow directly for the Salisbury chemical attack.

The four allies said it was "an assault on UK sovereignty" and a breach of international law that "threatens the security of us all".

But Jeremy Corbyn continued to be reluctant to place the blame firmly with the Kremlin, saying the evidence “appears” to show Russia was behind the attack, which, the Labour leader suggested might have been carried out by “mafia-like groups”.

Earlier, the Prime Minister visited the scene of the attack on the former Russian spy, Sergei Skripal and his daughter Yulia, to thank the emergency services for their professionalism and the people of Salisbury for their “incredible patience”.

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Mrs May spoke to the first policeman who attended the incident - Detective Sergeant Nick Bailey - at the local hospital, where he is being treated for exposure to the nerve agent. No 10 declined to give details of what they described as a “private conversation”.

In the medieval city’s Guildhall, she met police officers Alex Way and Alex Collins, who were the first to respond to the emergency call. Mr Collins said: "It was a routine call; two people on a bench, slumped over, which is nothing out of the ordinary."

Mrs May replied: "You had no idea what you were dealing with. Thank you, what you did was what police do day in and day out. A routine call and you don't know what's there. You did a great job."

Clearly buoyed by the show of solidarity from Western allies - at one point the PM engaged in a fist-bump with a member of the public during a walkabout - she said: “What is important in the international arena - and we have taken this into Nato, the United Nations, through into the European Union - is that allies are standing alongside us and saying this is part of a pattern of activity that we have seen from Russia in their interference, their disruption that they have perpetrated across a number of countries in Europe.

"This happened in the UK but it could have happened anywhere and we take a united stance against it," declared Mrs May.

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After his first keynote speech as Defence Secretary, Gavin Williamson accused the Kremlin of “atrocious and outrageous” behaviour, adding: “Frankly, Russia should go away and should shut up."

Boris Johnson, the Foreign Secretary, confirmed the UK would submit a sample of the nerve agent to the Organisation for Prohibition of Chemical Weapons for it to carry out its own tests.

In Moscow, Vladimir Putin met with his security chiefs to plan Russia’s response.

Sergei Lavrov, the Foreign Minister, told reporters a response to the UK’s actions, which includes the expulsion of 23 Russian diplomats, would receive a response “very soon, I promise you”.

In their joint statement Mrs May, US President Donald Trump, France's Emmanuel Macron and Germany's Angela Merkel said they "abhorred" the poison attack and shared the assessment that there was "no plausible alternative explanation" other than Russia being responsible.

The leaders of “the Quad,” as it was quickly termed, called on Moscow to answer all questions about the Salisbury incident and "live up to its responsibilities as a member of the UN Security Council to uphold international peace and security".

They went on: "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 and any such use by a state party is a clear violation of the Chemical Weapons Convention and a breach of international law. It threatens the security of us all."

In Brussels, the Nato states were briefed by Sir Mark Sedwill, the UK National Security Adviser, at a meeting of the North Atlantic Council.

Jens Stoltenberg, the military alliance's Secretary General, said the attack took place "against the backdrop of a reckless pattern of Russian behaviour".

He stressed: “We do not want a new Cold War and we do not want to be dragged into a new arms race...But let there be no doubt, Nato will defend all allies against any threat.”

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In Washington, Mr Trump told reporters in the White House: "It certainly looks like the Russians were behind it. We are taking it very seriously."

His words came as the US administration enforced its toughest measures yet against Moscow, imposing sanctions against five groups and 19 individuals, accusing them of staging a series of cyber-attacks on America’s nuclear, aviation and energy infrastructure.

Meanwhile at home, Mr Corbyn - under fire from Tory and Labour MPs over his quizzical response to the Conservative Government’s approach - warned against adopting a “McCarthyite intolerance of dissent,” urging the PM not to “rush way ahead of the evidence”.

But his critics continued to vent their anger at his stance with Tory backbencher Peter Bone branding him an “apologist for Russia”.