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 March 4.
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 on Tuesday.
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.