Russia will not respond to Theresa May's midnight ultimatum over the poisoning of a former spy in Salisbury, the Russian foreign ministry has said, unless it is given samples of the nerve agent used in the attack.
In a statement, the foreign ministry said British threats to punish Moscow over the poisoning would not go unanswered, and described the allegations of Russian involvement as a provocation.
The UK Prime Minister had given Russia until midnight on Tuesday to explain why a Soviet-era nerve agent, produced in Russia, was used in the attack on former spy Sergei Skripal and his daughter Yulia, who remain in a serious condition in hospital.
Ms May said the only two explanations for the finding were that the attack was ordered by the Kremlin, or that the Russian government had lost control of the military grade chemical weapon and let it fall into the hands of the perpetrators.
Russia insists it had nothing to do with the attack and said it will ignore the ultimatum to explain itself until London both hands over samples of the Novichok agent used and begins to comply with its obligations under the Chemical Weapon Convention, which oversees joint investigations of such incidents.
"Any threats to take 'sanctions' against Russia will not be left without a response," the foreign ministry said in a statement. "The British side should understand that."
Sergei Lavrov, the Russian Foreign Minister, said earlier on Tuesday that his country "is not to blame" for the poisoning of Sergei Skripal.