Britain gave President Vladimir Putin until midnight yesterday 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.
Sergei Skripal, 66, and his daughter Yulia, 33, have been in hospital in a critical condition since March 4 when they were found unconscious on a bench outside a shopping centre in the English cathedral city of Salisbury.
Prime Minister Theresa May said it was "highly likely" that Russia was to blame after Britain identified the substance as part of the highly-lethal Novichok group of nerve agents developed by the Soviet military in the 1970s and 1980s.
"Either this was a direct act by the Russian state against our country," May told parliament on Monday. "Or the Russian government lost control of this potentially catastrophically damaging nerve agent and allowed it to get into the hands of others."
Russia holds a presidential election on March 18 in which Putin, himself a former KGB spy, is expected to coast easily to a fourth term in the Kremlin. It has denied any role in the poisoning and says Britain is whipping up anti-Russian hysteria.
Russian ambassador Alexander Yakovenko, summoned to the Foreign Office, was given until the end of yesterday to explain what happened or face what May said were "much more extensive" measures against the $1.5 trillion Russian economy.
If no satisfactory Russian response is received by midnight London time then May will outline Britain's response in parliament.