The UK's threat of “punitive” measures against Russia over the poisoning of a former snoop and his daughter will “meet with a response”, Russia has said.
Russia's UK embassy posted a series of tweets as a deadline looms for it to explain how a Russian-made nerve agent was used in Salisbury.
It said there would be no reply to the “ultimatum” until it was given access to samples of the substance.
No 10 said Donald Trump has told the PM the “US was with the UK all the way”.
In a telephone conversation, the US president agreed with Theresa May that the Russian government “must provide unambiguous answers as to how this nerve agent came to be used” against Sergei Skripal, 66, and his daughter Yulia, 33, on 4 March. Downing Street added.
The PM has said it is “highly likely” Russia was involved in the poisoning and the UK must “stand ready” to take action.
She said details would be set out in the Commons on Wednesday should there be no “credible” explanation by midnight on Tuesday from Moscow.
No 10 said Mr May also spoke on the phone to France's President Emmanuel Macron and German Chancellor Angela Merkel, who both carried “solidarity” with the UK ‘s position.
Foreign Secretary Boris Johnson said if the attack was shown to be a “direct act” by the Russian state it would be a “clear violation of the chemical weapons convention, a breach of international law and a threat to those who abide by the rules-based international order”.
The Foreign Office said the UK would brief a conference of the North Atlantic Council, Nato's political decision-making body, on Wednesday.
What we know so far What are Novichok nerve agents ? The spy at centre of poison mystery Media captionVladimir Putin is challenged by the BBC over the Skripal poisoning Image caption Police continued to work on Tuesday evening near to where the Skripals were found collapsed Image caption Officers have appealed for information on Sergei Skripal's BMW automobile Up to 14 other demises to be re-examined for Russian links No change to England World Cup plans Media captionBoris Johnson: “We are dedicating Russia until midnight to explain” The Football Association says it has built no change to plans for England's presence at this year's World Cup in Russia despite calls to consider a boycott next following the poisoning of the Skripals. Home Secretary Amber Rudd announced that MI5 and police are to look into claims that as many as 14 demises on UK soil may be linked to Russia. Counter-terror police are resulting an investigation into the “unexplained” death of a human believed to be Russian businessman Nikolai Glushkov – but officers say there was no evidence connecting the demise to the incidents in Salisbury. Ofcom says it is currently considering whether Kremlin-funded channel RT should broadcast in the UK if Russian involvement is proven in the Salisbury attack. Moscow has threatened to expel British media outlets if it is stripped of the broadcasting licence, while RT said its output “continues to adhere to all standards” and Ofcom was “conflating its role” as a broadcasting regulator.
