British prime minister: 'Highly likely' Russia behind spy poisoning Sergei Skripal and his daughter were exposed to a military-grade nerve agent.

British Prime Minister Theresa May told members of the Parliament today that it was "highly likely" Russia was responsible for poisoning Russian ex-spy Sergei Skripal and his daughter in England earlier this month.

She said Skripal, 66, and his daughter Yulia Skripal, 33, were exposed to a "military-grade nerve agent of a type developed by Russia." They were found slumped over in Salisbury, England, last week, and remained in critical condition.

May told members of Parliament that British authorities had concluded the Skripals were exposed to an agent that was part of the "Novichok" group of nerve agents developed by Russia.

Because Russia had produced the agent in the past and "would still be capable of doing so," the British government had "concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal," May said.

"Either this was a direct act by the Russian state against our country," May added, "or the Russian government lost control of its potentially catastrophically damaging nerve agent and allowed it to get into the hands of others."

The British foreign secretary has summoned the Russian ambassador to the United Kingdom to the British foreign ministry "and asked him to explain which of these two possibilities it is," May said. The foreign secretary, Boris Johnson, has requested a response by the end of Tuesday.

"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," May said.

May laid out a history of what she called "Russian state aggression" -- from annexing the territory of Crimea from Ukraine to a recent speech by Russian President Vladimir Putin where he unveiled weapons systems.

"This attempted murder using a weapons-grade nerve agent in a British town was not just a crime against the Skripals," May said. "It was an indiscriminate and reckless act against the United Kingdom, putting the lives of innocent civilians at risk. And we will not tolerate such a brazen attempt to murder innocent civilians on our soil."

A spokesman for Russia’s foreign ministry, Maria Zakharova, called May’s accusation a “circus show in the British Parliament,” according to Russian news agency Interfax.

“The conclusion is obvious," Zakharova told reporters, Interfax reported. "It is the latest in a political-information campaign, based on provocation."

"Before composing new fairy tales," she said, the U.K. should explain previous cases of Russians who have died on British soil -- such as Alexander Litvinenko, a former Russian intelligence official and Putin opponent who was poisoned in London in 2006.

Russia's foreign ministry accused U.K. officials and the British media of trying to using Skripal in a premeditated campaign to discredit Russia before it hosts the FIFA World Cup this summer.

“We have warned more than once, that before the start of the soccer World Cup in Russia, Western media will launch a full-scale media campaign with the goal of discrediting Russia and undermining trust in it as host of this sporting event,” the ministry said in a statement.

Skripal was a retired Russian double agent living in the U.K. He was convicted in Russia in 2006 for spying for Great Britain and released in 2010 as part of a prisoner swap.

ABC News' Patrick Reevell contributed reporting from Moscow.