British Foreign Secretary Boris Johnson said that it was overwhelmingly likely that Russian President Vladimir Putin himself made the decision to use a military-grade nerve toxin against a former Russian spy in Britain.

"We have nothing against the Russians themselves. There is to be no Russophobia as a result of what is happening," Mr Johnson said.

"Our quarrel is with Putin’s Kremlin, and with his decision and we think it overwhelmingly likely that it was his decision to direct the use of a nerve agent on the streets of the UK, on the streets of Europe for the first time since the Second World War."

The Kremlin has described Mr Johnson’s comments as "shocking and unforgivable". Russia has denied any involvement in the poisoning of Sergei Skripal and his daughter in Salisbury.

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Kremlin spokesman Dmitry Peskov said mentioning Mr Putin in the context of Skripal's poisoning "is nothing but shocking and unforgiveable behaviour from the point of view of diplomacy". 

Tension between the West and Russia has heightened as Russia continues to plan its response to British Prime Minister Theresa May's expulsion of diplomats.

A tit-for-tat reaction is expected to her decision to kick out 23 diplomats who she said were undeclared intelligence officers.

Russia is also planning a response to the US after Donald Trump's administration imposed sanctions on Russians allegedly involved in interfering with the 2016 US elections and cyber-attacks.

The attack the Skripals was highlighted by the US Treasury as one of the justifications for the tougher line against Moscow.

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The US treasury department said the use of a military-grade nerve agent in the Salisbury incident "further demonstrates the reckless and irresponsible conduct of its (Russia's) government".

The sanctions prompted a swift threat of retaliation from the Russian government, which said a response was already being prepared.

Meanwhile, the Kremlin continued to consider how to respond to Mrs May after the largest expulsion of diplomats since the Cold War was announced on Wednesday.

Speaking at an event in Moscow last night, Mr Putin said Russia was a "proud" nation "and will be in the future, too".

Mr Putin had a meeting with his security council yesterday to consider UK-Russia relations.

Russian foreign minister Sergei Lavrov warned Moscow will expel British diplomats "soon".

He suggested that the "provocation with Skripal" was an attempt to distract attention from the Brexit process.

Mr Putin's spokesman Dmitry Peskov said the final decision on retaliatory measures "will, of course, be made by the Russian president".

He added: "There is no doubt that he will choose the variant that best of all corresponds to the interests of the Russian Federation."

In a demonstration of the West's unity, Mrs May and Mr Trump, along with Germany's Angela Merkel and France's Emmanuel Macron, issued a joint statement endorsing the prime minister's conclusion that it was "highly likely" Russia was behind the attack on the Skripals.

The Daily Telegraph reported that the nerve agent used in the attack could have been planted in Yulia Skripal's suitcase during a recent trip to Moscow.

The newspaper said senior intelligence sources believe an item of clothing, cosmetics or a gift could have been laced with the Novichok toxin.