By AFP

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British Prime Minister Theresa May said Monday it was highly likely that Russia was responsible for a "reckless and despicable" attack on a former double agent in Britain, an accusation rejected by Moscow as a "provocation".

In a dramatic escalation of diplomatic tensions over the attack on Sergei Skripal and his daughter on March 4, May said the nerve agent that poisoned them was a military-grade type developed by Russia.

She told parliament that Moscow had previously used this group of nerve agents known as Novichok, had a history of state-sponsored assassinations and viewed defectors such as Skripal as legitimate targets.

"The government has concluded that it is highly likely that Russia was responsible for the act against Sergei and Yulia Skripal," she told the House of Commons.

Moscow swiftly rejected May's comments to lawmakers, saying it was "a circus show" and an attempt to undermine trust ahead of its hosting of this summer's World Cup.

The prime minister's statement was part of "another information and political campaign based on provocation," said Russian foreign ministry spokeswoman Maria Zakharova in comments carried by news agencies.

In its first comments on the case Monday, the White House offered its support to Britain, calling the attack "an outrage".

But press secretary Sarah Sanders stopped short of referencing any possible Russian involvement.

"The attack was reckless, indiscriminate and irresponsible," she said.

Meanwhile French President Emmanuel Macron "offered his solidarity with the UK" in a phone call with May, Downing Street said.

"They discussed the wide pattern of aggressive Russian behaviour and agreed that it would be important to continue to act in concert with allies to address it," it added.

Deadline imposed

Earlier Monday, Foreign Secretary Boris Johnson summoned the Russian ambassador to demand a "full and complete disclosure" of the Novichok programme to the global chemical weapons watchdog, May said.

The envoy was asked to explain whether the attack on Skripal was a "direct act by the Russian state against our country", or if the government had "lost control" of the agent and someone else had used it, she added.

"He has requested the Russian government's response by the end of tomorrow (Tuesday)," the prime minister said.

May added if there was "no credible response" Britain will conclude it was "an unlawful use of force by the Russian State against the UK", and pledged to outline a "full range of measures" in response.

Interior minister Amber Rudd will chair a meeting of the government's emergency Cobra committee at 11:30am (1130 GMT) Tuesday for an update on the investigation, the Home Office said.

Skripal was jailed for selling Russian secrets to Britain but moved to this country in a spy swap in 2010, settling in the sleepy city of Salisbury in southwest England.

He and his daughter Yulia, 33, remain in a critical condition in hospital after being found unconscious on a bench outside a shopping centre in the city.

Pharmacology experts said Novichok is a "more dangerous and sophisticated agent" than sarin or VX.

"It causes a slowing of the heart and restriction of the airways, leading to death by asphyxiation," said Gary Stephens, a professor at Britain's University of Reading.

He added it was harder to identify and first developed because its component parts are not banned globally.

"It means the chemicals that are mixed to create it are much easier to deliver with no risk to the health of the courier," Stephens said.

'Brazen attempt to murder'

Johnson has previously said the case has "echoes" of the 2006 poisoning by radiation of ex-spy and Kremlin critic Alexander Litvinenko, which London has blamed on Moscow.

May noted that after Litvinenko's murder, which caused a deep rift in relations with Russia, Britain expelled Russian diplomats, suspended security cooperation and froze the assets of the suspects, among other measures.

"We must now stand ready to take much more extensive measures," she said, adding: "We will not tolerate such a brazen attempt to murder innocent civilians on our soil."

May had already suggested a boycott of the 2018 football World Cup by officials and dignitaries.

The bench where Skripal was found, his home and a pub and restaurant the pair visited have all been sealed off, as has the grave of Skripal's wife, Liudmila, who died in 2012.

England's chief medical officer, Sally Davies, on Sunday said that up to 500 people who may have come into minimal contact with the nerve agent should wash their clothes and belongings as a precaution.

Health authorities insisted the risk to the general public remains low and Davies was "confident" nobody else was harmed, but said there were concerns that prolonged exposure to these substances may cause health problems.