London - Britain's prime minister said on Monday that it's "highly likely" that Russia was responsible for poisoning ex-spy Sergei Skripal with a military-grade nerve agent in southwestern England last week.
Theresa May said that Russian state involvement, if proven, would be considered an "unlawful use of force by the Russian state against the United Kingdom" and would draw a strong response.
Shortly after May's remarks, Russia's Foreign Ministry dismissed suggestions it could be behind the poisoning as a "circus show".
May said that British scientists have determined that Skripal and his daughter were poisoned in Salisbury with Novichock - a form of nerve agent developed by the Soviet Union near the end of the Cold War - and there were two possible explanations.
"Either this was a direct act by the Russian state against our country, or the Russian government lost control of this potentially catastrophically damaging nerve agent and allowed it to get into the hands of others," she said.
May said that Britain had given the Russian ambassador in London a deadline of Tuesday to explain which version is true.
May spoke in the House of Commons after she chaired a National Security Council meeting to hear the latest evidence in the case. She has been under mounting pressure to hit Russia with sanctions, diplomatic expulsions and other measures in response to the poisoning, the latest in a string of mysterious mishaps to befall Russians in Britain in recent years.
May said Britain would consider tough action if Russia's explanation is inadequate, though she didn't give details.
She said Britain would be prepared to take "much more extensive measures" than the diplomatic expulsions and limited sanctions imposed after the death of former Russian agent Alexander Litvinenko, who was poisoned with radioactive tea in London in 2006.
"There can be no question of business as usual with Russia," she said.
"We will not tolerate such a brazen attempt to murder innocent civilians on our soil," May added.
Skripal, 66, and his 33-year-old daughter, Yulia, remain in critical condition following the March 4 nerve agent attack. A police detective who came in contact with them is in serious but stable condition.
The case has similarities to the killing of Litvinenko. A British inquiry concluded that his death was the work of the Russian state and had probably been authorised by Russian President Vladimir Putin.
The Kremlin has denied involvement in Litvinenko's death, and dismissed claims it was behind the attack on the Skripals.
Putin spokesperson Dmitry Peskov told reporters that Sergei Skripal worked for British intelligence and was poisoned on British soil, and therefore the incident "has nothing to do with Russia, let alone the Russian leadership". Peskov also said the Kremlin hasn't heard any official statements of Russian involvement.
Skripal was a Russian military intelligence officer when he was recruited to spy for Britain in the 1990s. He was jailed in Russia in 2006 for revealing state secrets before being freed in a spy swap in 2010. He had settled in the cathedral city of Salisbury, 140km southwest of London.
He and his daughter were found comatose on a bench near the city centre after visiting an Italian restaurant and a pub.
Almost 200 troops, including soldiers trained in chemical warfare and decontamination, have been deployed to Salisbury to assist the police investigation into where the nerve agent came from and how it was delivered.
British officials have said the risk to the public is low, but urged people who visited the Zizzi restaurant or the Mill pub to wash their clothes and take other precautions. Some have questioned why it took health authorities a week to issue the advice.