Prime Minister Theresa May speaking in the House of Commons in London about the Salisbury attack | PA Images Theresa May blames Russia for nerve gas attack PM said assassination attempt was either ordered by Russia or the Kremlin had lost control of the nerve agent used.

LONDON — Theresa May on Monday challenged President Vladimir Putin to explain how a Russian nerve agent came to be used on British soil — and must quickly decide her next move.

The U.K. prime minister accused Russia of being responsible for the attempted murder of double agent Sergei Skripal, who along with his daughter Yulia, was found unconscious in Salisbury, southern England, more than a week ago and remains in a critical condition in hospital.

Speaking in the House the Commons following a meeting of the U.K. government's National Security Council, the prime minister told MPs the government had concluded it was “highly likely” that Moscow was behind the poisoning because the nerve agent used is produced in Russia.

The attack is a significant security challenge for Theresa May and comes at a time when the U.K.'s relationships with many of its traditional allies are strained.

Almost a year after the prime minister began the process of pulling Britain out of the European Union, May has repeatedly stressed Britain wants to work closely with Europe on defense but is yet to agree what their future relationship will look like. On Monday evening, the prime minister spoke with French President Emmanuel Macron who offered his solidarity, according to a Downing Street spokesman.

The incident also tests the U.K.'s relationship with America. The White House was quick to offer the "fullest condemnation" of the attack Monday, describing it as "reckless, indiscriminate and irresponsible." According to the Associated Press, U.S. Secretary of State Rex Tillerson told reporters on a flight to the U.S. from Africa that the poison used in the attack "came from Russia," whether the Kremlin knew about it or not. He called the poisoning an “egregious act” that was “almost beyond comprehension” and "certainly will trigger" a U.S. response.

“I cannot understand why anyone would take such an action. But this is a substance that is known to us and does not exist widely,” Tillerson told reporters. “It is only in the hands of a very, very limited number of parties.”

Theresa May, who as home secretary resisted an open inquiry into the death in 2006 of another former Russian spy, Alexander Litvinenko, also faces domestic pressure to take more effective action against Russia. Litvinenko's widow Marina told Sky News Sunday "nothing was done" following the inquiry into her husband's death.

A spokesperson for the Russian foreign ministry described May's remarks as a "circus show," according to Russia Today. "The conclusion is obvious: This is another information and political campaign, based on provocation," said Maria Zakharova.

'Unlawful use of force'

Foreign Secretary Boris Johnson on Monday summoned the Russian ambassador to explain how the nerve agent had been used and gave him until the end of Tuesday to respond.

"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 told MPs.

Russia already faces economic sanctions by the EU and other nations, including the U.S., as punishment for Moscow’s 2014 annexation of Crimea and subsequent military intervention in eastern Ukraine.

However, the U.K. does not have the ability to impose sanctions unilaterally and therefore must work with EU allies — the very governments it is currently negotiating a divorce deal with.

NATO Secretary-General Jens Stoltenberg said Monday the alliance is in contact with the U.K. about the attack, amid speculation it could invoke its collective defense clause if Russia is found responsible. The decision to do so must be made by the North Atlantic Council, the alliance’s chief political body, and will likely depend heavily on the U.K.’s response, and any request it makes for assistance.

The Times reported the U.K. government was considering a cyber-counterattack, including the deployment of malware to attack Kremlin computer networks or websites publishing fake news.

May told MPs Monday that experts at the U.K's chemical weapons research center at Porton Down concluded the chemical used in the attack was part of a group of nerve agents known as "Novichok."

She said the assassination attempt was either directly ordered by the Russian state or the Kremlin had lost control of the nerve agent and allowed it to get into the hands of others. The prime minister added this attack should be understood "against a backdrop of a well-established pattern of Russian State aggression."

"It is now clear that Mr. Skripal and his daughter were poisoned with a military-grade nerve agent of the type developed by Russia," May said. "The government has concluded it is highly likely that Russia was responsible for the act against Sergei Skripal."

Labour caution

In response to May's statement in the Commons, Labour leader Jeremy Corbyn urged caution and warned the prime minister not to take any action which would reduce “dialogue” with Moscow.

He also raised the amount of money the Conservative Party had received in donations from Russians in the U.K. prompting shouts of “disgrace” from Tory MPs.

“We need to continue seeking a robust dialogue with Russia on all the issues dividing our countries, both domestic and international — rather than simply cutting off contact and simply letting tensions and divisions get worse,” Corbyn said.

A series of Labour MPs spoke of the need for national unity and the need for a tough response. Labour MP John Woodcock said: "It would put our national security at risk if we were led by anyone who did not understand the gravity of the threat that Russia poses to our nation."