The Russian Foreign Ministry said on Tuesday that British threats to target Moscow with punitive sanctions over the poisoning of a former spy would not be left unanswered and said false allegations of Russia's involvement were a provocation.

Britain gave President Vladimir Putin until midnight on Tuesday to explain how a nerve agent developed by the Soviet Union was used to strike down a former Russian double agent who passed secrets to British intelligence.

Russia says it had nothing to do with the attack and will ignore the ultimatum until London hands over samples of the nerve agent used and begins to comply with its obligations under the Chemical Weapon Convention, which foresees 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."