UK Prime Minister Theresa May told Parliament on Monday that it was "highly likely" Russia was behind the poisoning of a Russian former spy and his daughter on British soil.

The Russian government, which has denied any involvement in the incident, responded by dismissing May's speech to lawmakers as a "circus show."

Sergei Skripal, 66, and daughter Yulia, 33, are in critical but stable condition after being found March 4 unconscious on a park bench in Salisbury, England. The police officer who discovered them also remains hospitalized.

British authorities have determined the Skripals were poisoned by a military-grade nerve agent of a type known as "novichok" that the Russian government is known to have produced in the past, the prime minister said in the House of Commons.

Moscow "would still be capable" of producing the substance used in the attack, she said.

The government's conclusion drew on the findings of the Defense, Science and Technology Laboratory in Porton Down, some 13 km (8 mi) from Salisbury, which conserves nerve agents such as Sarin and VX "to help develop effective medical countermeasures and to test systems."

"Russia's record of conducting state sponsored assassinations and our assessment that Russia views some defectors as legitimate targets for assassination, the government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal," May said.

Skripal is a former Russian military intelligence officer who was convicted in 2004 of treason against Russia for spying on behalf of the United Kingdom's MI6 foreign intelligence agency.

He spent years behind bars in Russia before being released and exiled in 2010 as a part of a swap for Russian spies held in the UK and the United States.

"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 to get it into the hands of others," the Conservative prime minister said.

May said she would give the Russian government until Wednesday to provide an explanation.

"Should there be no credible response, we will conclude that this action amounts to an unlawful use of force by the Russian state against the United Kingdom," she said, adding that London was prepared to take "much more extensive measures" against Russia than previously.

Moscow's initial response to the accusation came within hours.

"It is a circus show in the British Parliament," Foreign Ministry spokeswoman Maria Zakharova said. "The conclusion is obvious: It's another political information campaign, based on a provocation."

The deputy chair of the international relations committee of the Russian Senate said the allegations were aimed at his country's upcoming presidential election or even the 2018 World Cup soccer tournament, set to begin in Russia in June.

"I think this a provocation on the eve of the elections and the World Cup," Vladimir Dzhabarov said. "Why eliminate Skripal? If he was such a valuable agent, he wouldn't have been swapped."