LONDON — Prime Minister Theresa May said Monday that British investigators have concluded it was “highly likely” that Russia was responsible for the poison attack that left a former Russian double agent and his daughter comatose on a park bench last week.

The British leader said police identified the poison as a “military-grade nerve agent of a type developed by Russia.”

She said Russia either engaged in a direct attack against Britain or lost control of the nerve agent it developed. Britain will not tolerate such a “brazen attempt to murder innocent civilians on our soil,” she warned.

As she addressed the House of Commons, the British leader stopped short of announcing retaliatory actions, saying she would give Russia a chance to respond to her government’s findings and would return to Parliament on Wednesday with a plan for specific action.

But in her remarks, May described a “reckless” and “indiscriminate” attack, which not only endangered the lives of its two principal victims, Sergei Skripal, 66, and his daughter, Yulia, 33, but potentially exposed scores of others, including a police officer who remains hospitalized.

Skripal was jailed in Russia in 2006 for selling state secrets to British intelligence for 10 years, but he was released in 2010 as part of a high-profile spy swap. His daughter has been living in Russia but has also spent long periods in England.

May strongly signaled that the already frosty relations between Britain and Russia were headed toward lows perhaps not seen since the Cold War. Lawmakers in Parliament called for sanctions and condemnations of Russia from the United Nations, European Union and United States.

Immediately after May’s remarks, the Russian government denounced her speech as a spectacle designed to mislead.

“It is a circus show in the British Parliament,” the Tass news agency quoted Russian Foreign Ministry spokeswoman Maria Zakharova as saying.

“The conclusion is obvious: It’s another political information campaign, based on a provocation.”

At a news briefing in Washington, White House press secretary Sarah Huckabee Sanders called the attack “an outrage” and said that “right now we are standing with our U.K. ally.” But she declined to say whether the United States shares the British assessment implicating Russia, and she did not mention Russia in her replies to questions about the attack.

The assault on a former Russian agent and his daughter was as public as could be.

British authorities were forced to cordon off a restaurant and pub near where the pair was found in downtown Salisbury, a quiet medieval town in southern England, best known for its nearby ruins, Stonehenge.

Over the weekend, days after the initial attack on March 4, British public health officials advised anyone who had patronized the businesses during a two-day window to wash their clothes, double-bag articles for dry cleaning and wipe down items such as jewelry.

They assured the public that the danger was “minimal,” but the specter of a nerve agent wafting around a pub created a wave of anger and unease.

During her question-and-answer session in Parliament, members of May’s government and the opposition took turns denouncing the attack as a “murderous” assault “with impunity” by a “Russian mafia state.”

May promised it would not be “business as usual” and that by Wednesday her government would offer detailed measures, depending on what the Russians said.

May said British investigators have concluded that the chemical used in the attack was part of a group of Russian nerve agents known as “Novichok.”

“Based on the positive identification of this chemical agent by world-leading experts at Porton Down, our knowledge that Russia has previously produced this agent and would still be capable of doing so, Russia’s record of conducting state-sponsored assassinations, and our assessment that Russia views some defectors as legitimate targets for assassinations, the government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal,” she said.

Novichok was developed in Moscow in 1987 at the State Union Scientific Research Institute for Organic Chemistry and Technology. That government laboratory was described by one of its top officials in the 1990s as “the leader in the technology of chemical destruction.”

The Soviet Union, under Mikhail Gorbachev, renounced the development and use of chemical weapons, but research continued in secret. In 1992, a scientist named Vil Mirzayanov, in interviews with the Moscow News and the Baltimore Sun, disclosed the existence of the chemical weapons program. The Sun confirmed later the existence of an agent then called “Novichok No. 5.” American chemical weapons experts had been unaware of its existence.

In 2000, under an agreement with the United States, a joint program to dispose safely of all of Russia’s chemical weapons stocks was launched. It is unlikely that this program succeeded in eradicating all chemical weapons.

May said she instructed Foreign Secretary Boris Johnson to demand that Moscow “immediately provide full and complete disclosure” of the Novichok program to the Organization for the Prohibition of Chemical Weapons.

Mathieu Boulegue, a research fellow with the Russia and Eurasia program at Chatham House, a London-based think tank, said he doubted that Russia would provide any detail, beyond denials. But the two-day pause was likely designed as “a way for the British government to prepare everyone for a robust response.”

He said it was difficult to know what Britain’s response would be exactly, “but there could be a signaling and testing of Russia’s pain threshold.”

Several lawmakers suggested that Britain pass its own version of America’s 2012 Magnitsky Act, which allows the U.S. government to impose sanctions, seize assets and blacklist foreign officials implicated in human rights abuses.

President Vladimir Putin’s government has strongly denied any involvement in the case and has responded by accusing Britain of stirring anti-Russian hysteria.

A BBC reporter quoted Putin as saying Monday, “Get to the bottom of things there, then we’ll discuss this,” when asked about the alleged poisoning.

Before May’s address, the Russian Embassy in London said: “We are outraged by the anti-Russian media campaign, condoned by the government, that influences the investigation and has a psychological effect on British residents.”

In its statement, Russia warned that the British government was playing “a very dangerous game” with public opinion and that this “unhelpful political track . . . also bears the risk of more serious long-term consequences for our relations.”

The Russian Embassy said that Russian nationals and others living in Britain are worried about their future in the country and that Russian journalists based in Britain are receiving threats.

On Monday, a popular anchor on Russian state TV accused Britain of masterminding the poisoning of the former spy and his daughter to undermine Russia as it prepares to host of the soccer World Cup tournament this summer.

Dmitry Kiselyov, the broadcast journalist, suggested it was all a plot to ruin the games for Russia and get British allies to boycott the World Cup.

“Why not poison him?” said Kiselyov. “Is he so valuable? And do it with his daughter to turn it into a real tear-jerker for the public.”

The Washington Post’s Will Englund in Washington and Karla Adam in London contributed to this report.