GETTY Russia v UK: Putin has rejected the Prime Minister's ultimatum

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Prime Minister Theresa May has given Russia until midnight to explain how a Soviet-era nerve agent was used against a former Russian double agent, and US President Donald Trump said he would condemn Russia if British evidence incriminated Moscow. 66-year-old Skripal and his 33-year-old daughter Yulia are fighting for their lives after being attacked with a nerve agent in Salisbury on March 4. A British policeman who went to the aid of Skripal was also affected by the nerve agent. He is now conscious in a serious but stable condition. Britain has identified the substance as belonging to the lethal Novichok group of nerve agents developed by the Soviet military in the 1970s and 1980s. This is a developing story. Express.co.uk will bring you the latest developments as they happen below.

8.47pm: Counter-terror police launch probe into Russian exile death A probe has been launched by counter-terrorism police amid reports a Russian exile, Boris Berezovsky, who was a close friend of Putin critic Boris Berezovsky has been found dead. Scotland Yard said a man in his 60s was found at a home in Clarence Avenue, New Malden, south-west London on Monday and that the cause of his death is unexplained. The force said in a statement: "At this stage the Met Police Counter Terrorism Command is leading the investigation as a precaution because of associations that the man is believed to have had. "There is no evidence to suggest a link to the incident in Salisbury." 8.04pm: The Russian Embassy has demanded an explanation over the cyber attack threat The Embassy has made a formal request to the Foreign Office for an explanation about suggestions it will be hit by a cyber strike. It tweeted: "Embassy has officially requested explanations from @foreignoffice on the cyber attack threats heard at Parliament and in the media. Russia takes a serious view on cyber security breaches." 7.06pm: World unites in condemnation of Salisbury attack Jens Stoltenberg, Secretary-General of the US-led NATO alliance, said the Salisbury attack was "horrendous". The European Union, which denounced the attack as "shocking”, offered help to track down those responsible. Donald Trump's endorsement of Britain's tough line, made during a phone call with May, came on the heels of messages of solidarity from France's President Emmanuel Macron and German Chancellor Angela Merkel. They gave the British Prime Minister additional hope of marshalling Western backing for her government as it heads towards a showdown with Putin.

6.37pm: Russian spy poison probe - police search for Sergei Skripal's red BMW Police are appealing for witnesses who may have seen former double agent Sergei Skripal and his daughter Yulia in a red BMW on the day they were found slumped on a bench in Salisbury town centre. In an update today, Met Police said 66-year-old Mr Skripal and his 33-year old daughter remain in a critical condition after being exposed to a nerve agent in a targeted attack on March 4. Officers are searching for witnesses who saw the former Russian spy and his daughter in their red BMW with the registration plate HD09 WAO between 1pm and 1.45pm on March 4 to contact police. 6.21pm: Rex Tillerson warns Russia is becoming increasingly isolated The ex-Secretary of State said if Russia continues on its current path, Moscow will become increasingly isolated. 6.17pm: Russian Embassy in London stresses Moscow is not involved in the Salisbury incident The Russian Embassy has fired off a series of tweets as tensions soar and Russia refuses to bow to London’s ultimatum until samples of the nerve agent are handed over. On Twitter, the Embassy said: “1/7 UK Ambassador Laurence Bristow was summoned to @MFA_Russia, 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. “2/7 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 4 March, 2018. “3/7 Moscow will not respond to London’s ultimatum until it receives samples of the chemical substance to which the UK investigators are referring. “4/7 Britain must comply with the Chemical Weapons Convention which stipulates joint investigation into the incident, for which Moscow is ready. “5/7 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. “6/7 Any threat to take “punitive” measures against Russia will meet with a response. The British side should be aware of that. “7/7 Today the Embassy sent a note to @ForeignOffice reiterating that Russia is not involved in the Salisbury incident and outlining the above mentioned demands for joint investigation.” 6.05pm: Donald Trump says US is with Britain US President Donald Trump said the United States was with Britain "all the way" in a telephone call with Prime Minister Theresa May. A Downing Street spokesman said: ”President Trump said the US was with the UK all the way, agreeing that the Russian government must provide unambiguous answers as to how this nerve agent came to be used.”

EPA Sergei Skripal is in a critical condition after being attacked with a nerve agent

The Russian government must provide unambiguous answers as to how this nerve agent came to be used Downing Street

5.42pm: Germany stands in “full solidarity” with Britain German Chancellor Angela Merkel told the Prime Minister she stood in "full solidarity" with the UK when they spoke by telephone on Tuesday afternoon. A Downing Street spokesman said: "The Prime Minister set out the conclusion reached by the UK Government that it was highly likely that Russia was responsible for the attack against Sergei and Yulia Skripal. "They discussed the pattern of aggressive Russian behaviour and agreed it would be important to act in unison with allies to counter it. "Chancellor Merkel condemned the attack and said she stood in full solidarity with the UK. "They agreed that the international community should coordinate closely as the investigation developed and in the wake of Russia's response." 5.39pm: French President Emmanuel Macron condemnns “unacceptable” nerve agent attack in Britain. Mr Macron expressed solidarity with the UK. 5.30pm: Moscow rejects May’s ultimatum The Russian Embassy in the UK has said Moscow "will not respond to London's ultimatum" until it is given access to samples of the nerve agent used in the attempted murder of Sergei and Yulia Skripal. After the UK Government said it was considering a range of sanctions, the Embassy added: "Any threat to take 'punitive' measures against Russia will meet with a response. The British side should be aware of that."

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