London: The UK government today said that it was "encouraged" by the support offered by its international allies over the poisoning of a UK-based Russian spy, even as Russia denied any role in the deadly nerve agent attack.

UK foreign secretary Boris Johnson said that he had spoken to a number of allies, including the US and members of the North Atlantic Treaty Organisation (NATO), and there had been a "willingness" to show "solidarity". His comments came a day after British Prime Minister Theresa May set a Tuesday midnight deadline for the Russian government to clarify how much it knew about the poisoning of Sergei Skripal and his daughter in Salisbury on March 4. In a House of Commons statement yesterday, she said that it is "highly likely" that Russia was involved in the "brazen attempt to murder innocent civilians on our soil" with a military-grade nerve agent of a type developed by the country. "Either this was a direct action by the Russian state against our country, or the Russian government lost control of its potentially catastrophically damaging nerve agent and allowed it to get into the hands of others," she said. The poison used against Skripal and his daughter was revealed as part of a group of deadly nerve agents known as 'Novichok', which means "newcomer" and was used by the Soviet Union during the Cold War era in 1970s and 1980s.

Like most nerve agents, it has the effect of blocking messages from the nerves to the muscles in the body, which leads to a collapse of body functions and ultimately death by asphyxiation. Russian foreign minister Sergei Lavrov said that Russia was "not to blame" and demanded access to samples of the substance used against Skripal. He said Moscow was willing to cooperate with the investigation but the UK would be "better off" complying with its international obligations "before putting forward ultimatums".

"Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian State against the United Kingdom," May had told Parliament. International allies have been responding to the nerve agent attack, offering support to the UK against the Kremlin. US Secretary of State Rex Tillerson, who spoke to Johnson on the phone about the case today, said the US supported the UK's assessment that Russia was likely responsible. He said it appeared the "really egregious act... clearly came from Russia" and there should be "serious consequences".

"We agree that those responsible – both those who committed the crime and those who ordered it – must face appropriately serious consequences. We stand in solidarity with our allies in the United Kingdom and will continue to coordinate closely our responses," he said. NATO secretary-general Jens Stoltenberg said: "The UK is a highly valued ally, and this incident is of great concern." May also spoke to French President Emmanuel Macron on Monday and the two leaders "agreed that it would be important to continue to act in concert with allies" to address what it called "the wide pattern of aggressive Russian behaviour", a Downing Street spokesperson said.

An inevitable tightening of sanctions against Russia now seem inevitable as relations between London and Moscow continue to deteriorate. Russia's ambassador to the UK, Alexander Yakovenko, has until midnight on Tuesday to provide a "credible response". In her Commons statement, Theresa May made a specific reference to the use of radiological substances in the "barbaric assault" on Alexander Litvenenko – another Russian spy murdered 11 years ago in London. Following Litvinenko's death, the UK had expelled Russian diplomats, suspended security cooperation, broke off bilateral plans on visas, froze the assets of the suspects and put them on international extradition lists.

May said that while those measures remain in place, "we must now stand ready to take much more extensive measures". On Sunday, Britain's public health authorities had asked hundreds of people who were at a restaurant and pub linked with the poisoning of the Russian spy to clean up their possessions to remove any traces of the deadly nerve agent. Trace amounts of the substance used to poison Skripal and his daughter were found at the Mill pub and Zizzi Italian restaurant in Salisbury, where the duo had been before they were found collapsed on a bench.

Skripal, 66, and his daughter Yulia, 33, both remain critically ill but in a stable condition in hospital. Detective Sergeant Nick Bailey, who was the first police officer to come in contact with the circumstances, also remains seriously ill in hospital but has recovered slightly. Colonel Skripal was convicted of treason in 2006 and jailed for 13 years for selling secrets to MI6, which had recruited him in the 1990s. The senior intelligence officer with Russian military intelligence GRU, was pardoned in a spy swap in 2010 and settled in Salisbury, Wiltshire.