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In a series of tweets the Russian embassy in the UK said: “Moscow will not respond to London’s ultimatum until it receives samples of the chemical substance to which the UK investigators are referring. “Britain must comply with the Chemical Weapons Convention which stipulates joint investigation into the incident, for which Moscow is ready. “Without that, there can be no sense in any statements from London. The incident appears to be yet another crooked attempt by the UK authorities to discredit Russia. “Any threat to take ‘punitive’ measures against Russia will meet with a response. The British side should be aware of that. “Today the Embassy sent a note to the Foreign Office reiterating that Russia is not involved in the Salisbury incident and outlining the above mentioned demands for joint investigation.” Russian Foreign Ministry spokeswoman Maria Zakharova said: "Who does Britain think it is, issuing ultimatums to a nuclear power?. “Not a single British media outlet will work in our country if they close Russia Today.”

GETTY•EPA Putin's envoys in the UK have refused to respond to Britain's ultimatum over the Russian spy attack

The embassy added: “UK Ambassador Laurence Bristow was summoned to Russia’s ministry of foreign affairs, where first deputy FM Vladimir Titov strongly protested the evidence-free accusations by the UK authorities of Russia’s alleged involvement in the poisoning of Sergei Skripal and his daughter Yulia. “It was stated that the actions of the UK authorities are a clear provocation and that the Russian Federation was not involved in the incident that took place in Salisbury on March 4, 2018." Mr Skripal, a former Russian spy turned MI6 agent, and his daughter are in a critical condition after they were found unconscious in Salisbury, Wiltshire, shortly after 4pm on Sunday, March 4. Following the incident Metropolitan Police Assistant Commissioner Mark Rowley, said: "We have to be alive to the fact of state threats." In a phone call with Theresa May today, Donald Trump said the US is “with the UK all the way” over investigation into the poisoning of Sergei Skripal and his daughter Yulia.

Russia spy poisoning: Latest pictures after Sergei Skripal puts Salisbury on lockdown Thu, March 8, 2018 Russia spy poisoning: Latest pictures after Sergei Skripal puts Salisbury on lockdown. Play slideshow PA 1 of 20 Personnel in hazmat suits secure a tent covering a bench in Salisbury's Maltings shopping centre

Downing Street added that the US President also agreed the Russian Government “must provide unambiguous answers as to how this nerve agent came to be used”. According to Whitehall sources Britain will consider implementing a version of the USA’s Magnitsky Act, which lists Russians involved in corruption and human rights abuses, and banning them from entering the UK. Fresh sanctions against the Kremlin are likely too – as well as beefed up deployments elsewhere in Eastern Europe. Mrs May might also deploy a specialist cyber unit in Britain in order to attack Kremlin computer networks and troll factories. Initially Russia said it had no information regarding the "tragic situation" and claimed it was willing to co-operate.

EPA Former Russian spy Sergei Skripal and his daughter are in a critical condition following the attack