Up to 20 EU countries and the United States are preparing to expel Russian diplomats almost three weeks after the poisoning of Russian double agent Sergei Skripal.
The co-ordinated response will begin on Monday when the European Union recalls its ambassador to Moscow, The Times has reported.
It comes more than a week after the UK expelled 23 Russian diplomats it labelled "undeclared intelligence agents".
The countries set to participate in the response include France, Germany, Poland, Ireland, the Netherlands, Estonia, Latvia, Lithuania, Bulgaria, the Czech Republic, Denmark and nine other European states.
In a major coup for Theresa May, it appears that the United States will also participate after President Donald Trump's National Security Council on Friday night recommended the expulsion of a number of diplomats. The president is set to decide on implementing the recommendation next week.
Friday saw European leaders issue a tough joint statement pinning the blame for the poisoning in Salisbury on Russia. "It agrees with the United Kingdom government's assessment that it is highly likely that the Russian Federation is responsible and that there is no plausible alternative explanation. We stand in unqualified solidarity with the United Kingdom in the face of this grave challenge to our shared security," the statement read.
The statement was the result of days of discussions between the British prime minister and European leaders, with Mrs May even sharing sensitive intelligence on Novichok, the nerve agent reportedly used in the attack, in an effort to persuade European allies to back a robust, unified response.
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[ EU nations say they will act against Russia over UK spy poisoning ]
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However, there remain obvious divisions over Russia within the European community. Notably absent from the response were Greece and Italy, two EU states who are widely seen as advocates of a closer relationship between Russia and the European Union.
The diplomatic development comes as it emerges that Mr Skripal had reportedly begged Russian President Vladimir Putin to pardon him and permit him to return to the country to visit family and friends.
Speaking to the BBC, Vladimir Timoshkov said his friend Mr Skripal had written to the Russian president insisting he was not a traitor. "In 2012 he called me. We spoke for about half an hour. He called me from London. He denied he was a traitor ... [he told me] he wrote to Vladimir Putin asking to be fully pardoned and to be allowed to visit Russia. His mother, brother and other relatives were [in Russia]."
Mr Skripal said he denied being a traitor on the grounds that he had pledged loyalty to the Soviet Union, not Russia. "[The] oath he had sworn was to his socialist Motherland, the Soviet Union, not Russia," Mr Timoshkov told the BBC.
He added the Mr Skripal said he regretted becoming a double agent "because his life had become all messed up".
The Kremlin denied receiving a letter.
The revelation raises further questions in the debate as to whether or not Mr Skripal had been working with British intelligence since arriving in the UK in 2010. Speaking last week at the Frontline Club in London, Marina Litvinenko questioned the idea that he had ceased all intelligence work. "I don't think he was a quiet pensioner."
Mr Skripal and his daughter Yulia remain in a critical condition in hospital following the poisoning on March 4. Det Sgt Nick Bailey, the police officer who was hospitalised after responding to the incident, was released on Thursday.