British Prime Minister Theresa May, speaking to lawmakers about the poisoning of a former Russian double agent and his daughter, said it was "highly likely" that Russia was behind the attack. Sergei Skripal, the Russian who spied for Britain, and his 33-year old daughter Yulia remained in critical condition on Monday.

May said the investigation found that Skripal and his daughter were poisoned by a military grade nerve agent developed by Russia, known as a Novichok agent.

"Russia has previously produced this agent, and the government has concluded that it is highly likely that Russia was responsible," May said.

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May said there were two possible explanations for the attack: "either it was a direct act by Russia against our country or Russia lost control" of the powerful nerve agent.

Should Russian state involvement be proven, May said, it would be considered an "unlawful use of force by the Russian state against the United Kingdom." She gave the Russian ambassador until the end of Tuesday to respond.

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 the Russian explanation is inadequate, though she didn't give details. "There can be no question of business as usual with Russia," she said.

The case has similarities to the killing of former Russian spy Alexander Litvinenko, who was poisoned with radioactive tea in London in 2006. A British inquiry concluded that his death was the work of the Russian state and had probably been authorized by Russian President Vladimir Putin.

The Kremlin has rejected suggestions that it's behind the poisoning.

Putin spokesman 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.

CBS News correspondent Charlie D'Agata reported that forensics and decontamination work was still underway Monday at an Italian restaurant in Salisbury where Skripal and his daughter visited before collapsing on a bench nearby.

Police officers stand on duty outside a pub which has been secured as part of the investigation into the poisoning of former Russian intelligence agent Sergei Skripal and his daughter Yulia, in Salisbury, Britain March 12, 2018. Henry Nicholls / REUTERS

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, 90 miles southwest of London.

He and his daughter were found comatose on a bench near the city center 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.

Andy Harder, 63, who works in a coin and stamp collector's shop in Salisbury, had been in the Mill pub the day after the Skripals were attacked, and before police cordoned off the area.

Harder said he washed his jacket off with an antiseptic cleaner after authorities gave the guidance Sunday.

"So I've washed all my clothes, I've taken my jacket and done that with Dettol -- I mean I don't know what to use, really," he said. "That's supposed to kill most things. I've had a good scrub up, so it should be OK."