Moscow on Tuesday denied it was behind the poisoning of a former double agent in Britain as a midnight deadline loomed to explain how a suspected Russian-made nerve agent was used in the attack.
"Russia is not guilty," said foreign minister Sergei Lavrov of the attack on Sergei Skripal and his daughter in an English city on 4 March.
The United States, NATO and the European Union have all backed Britain in the deepening diplomatic crisis.
Lavrov said Russia was "ready to cooperate", but complained Britain had rejected its requests for "access" to the nerve agent samples.
British prime minister Theresa May told parliament it was "highly likely" Russia was behind the poisoning, giving Moscow until the end of Tuesday to respond.
In a cabinet meeting on Tuesday, she called the attack "reckless, indiscriminate and despicable", adding she would inform MPs of her response on Wednesday.
Ahead of the announcement, Britain's National Security Council will meet "to discuss the response from Russia, whatever that may be," said May's spokesman.
Foreign secretary Boris Johnson vowed that Britain's response, if it concludes Russia was responsible, would be "commensurate".
May has said that her government was considering a British version of the US "Magnitsky Act", which was adopted in 2012 to punish Russian officials accused of human rights violations.
Broadcasting regulator Ofcom has also warned Russian state-owned news channel RT that its licence in Britain could be reviewed.
British police and intelligence services are also to revisit the deaths of 14 people on its soil that may be linked to Russia, Home Secretary Amber Rudd said Tuesday.
- 'They will not recover' -
Skripal, 66, and his daughter Yulia, 33, remain in a critical condition in hospital after being found unconscious on a bench outside a shopping centre in the southwestern city of Salisbury.
Emergency workers in biohazard suits have been deployed in the normally sleepy city, while about 500 people who may have come into minimal contact with the nerve agent were urged to wash clothes and belongings as a precaution.
May told British lawmakers that Moscow had previously used a group of nerve agents known as Novichok, had a history of state-sponsored assassinations and viewed defectors such as Skripal as legitimate targets.
She demanded Moscow disclose details of its development of the Novichok nerve agents programme to the Organisation for the Prohibition of Chemical Weapons (OPCW).
Vil Mirzayanov, a chemist who worked on the Novichok programme and now lives in the United States, was quoted as saying that the nerve agent's effects were "brutal".
"These people are gone -- the man and his daughter. Even if they survive they will not recover," he was quoted as saying.
US President Donald Trump said he would "condemn Russia or whoever" was responsible for the attack, while European Commission Vice President Frans Timmermans said the EU was united in "unwavering" solidarity.
- 'Dirty attempt' -
NATO Secretary General Jens Stoltenberg said the incident was "of great concern", as the Daily Telegraph newspaper reported that Britain was consulting NATO allies about possibly invoking its Article 5 principle of common defence.
But Russian president Vladimir Putin brushed aside questions about Moscow's involvement in the attack on Monday, telling the BBC: "Sort things out from your side and then we will discuss this with you."
Moscow on Tuesday summoned the British ambassador and called the accusations "another dirty attempt by British authorities to discredit Russia."
British counter-terrorism chief Neil Basu called the investigation "extremely challenging" while giving more details on Tuesday.
He revealed that Yulia had arrived in Britain on March 3, the day before the pair visited a pub and an Italian restaurant, ahead of their collapse.
The police chief said he was particularly interested in witnesses who saw the pair in Skripal's red BMW between 1pm (1300 GMT) and 1.45pm on Sunday, on their way into town.
Medical professionals treated 38 people in total, he said, 34 of whom were released while one person continues to be monitored.
Police officer Nick Bailey, one of the first to the scene, remains "serious but stable", but is "making good progress", said Basu.
- 'Sophisticated' poison -
Skripal, an ex-military intelligence officer who was jailed for selling Russian secrets to London, moved to Britain in a spy swap in 2010, settling in Salisbury.
Pharmacology experts said Novichok, a broad category of more than 100 nerve agents developed by Russia during the late stages of the Cold War, was "more dangerous and sophisticated" than sarin or VX.
The BBC reported that investigators now believe the nerve agent may have been deployed in powder form through the ventilation system of Skripal's car.
The attack carried echoes of the 2006 radiation poisoning of former Russian agent Alexander Litvinenko.
A British inquiry concluded in 2016 that Litvinenko's killing was "probably" approved by President Vladimir Putin.