Britain warns Russia after former double agent mysteriously falls ill in U.K.

Britain warned onTuesday it would respond robustly if Russia was shown to bebehind the mysterious illness that struck down a former Russiandouble agent convicted of betraying dozens of spies to Britishintelligence.

Foreign Secretary Boris Johnson named Sergei Skripal, once acolonel in Russia's GRU military intelligence service, and hisdaughter, Yulia, as the two people who were found unconscious onSunday on a bench outside a shopping centre in southern England.

Skripal, 66, and his 33-year-old daughter were exposed towhat police said was an unknown substance in the English city ofSalisbury. Both are still critically ill in intensive care.

"We don't know exactly what has taken place in Salisbury,but if it's as bad as it looks, it is another crime in the litany of crimes that we can lay at Russia's door," Johnson told Parliament in London.

"It is clear that Russia, I'm afraid, is now in manyrespects a malign and disruptive force, and the U.K. is in the lead across the world in trying to counteract that activity."

If Moscow was shown to be behind Skripal's illness, Johnsonsaid, it would be difficult to see how U.K. representation couldgo to the World Cup in Russia in a normal way. A governmentsource said that meant attendance of ministers or dignitaries.

A statement posted to the Russian embassy in London's website said the events "as described by the British media, causes serious concern."

"Although U.K. law enforcement agencies have not given any substantive comments on this incident, media reports create an impression of a planned operation by the Russian special services, which is completely untrue," the statement attributed to a press officer said.

The embassy said its officials are seeking further detail from the British Foreign Office.

Previous poisoning case

A previous British inquiry said President Vladimir Putinprobably approved the 2006 murder of ex-KGB agent Alexander Litvinenko with radioactive polonium-210 in London. The Kremlinhas repeatedly denied any involvement in Litvinenko's killing.

Litvinenko, 43, an outspoken critic of Putin who fled Russiafor Britain six years before he was poisoned, died after drinking green tea laced with the rare and very potentradioactive isotope at London's Millennium Hotel.

It took weeks for British doctors to discern the causeof Litvinenko's illness.

His murder sent Britain's relations with Russia to what wasthen a post-Cold War low. Relations suffered further fromRussia's annexation of Crimea and military backing of SyrianPresident Bashar al-Assad against rebels trying to topple him.

Keir Giles, the director of the Conflict Studies Research Centre in Cambridge, England, said he "would be surprised if this were not linked back to Russia in some direct way."

It's hard not to see a pattern of the attacks becoming more and more brazen.- Keir Giles, director of Conflict Studies Research Centre

He said he could not rule out an overdose or some other kind of accidental poisoning but found it hard to picture such a scenario "that would lead to a full-scale decontamination of the street and the hospital."

Giles also invoked a string of suspicious deaths of Russian government opponents in Britain since Litvinenko's slaying.

"It's not just Litvinenko," he said. "It's hard not to see a pattern of the attacks becoming more and more brazen."

Counterterrorismpolice involved

London's Metropolitan Police said Tuesdaythe Counterterrorism Policing Network will lead the investigation because "it has the specialistexpertise" to deal with the unusual case.

While the British authorities said there was no known riskto the public from the unidentified substance, they sealed off the area where Skripal was found, a pizza restaurant calledZizzi and the Bishop's Mill pub in the centre of Salisbury.

Some investigators at one point wore yellow hazardous material suits,though most police at the scene did not.

Skripal, who passed the identity of dozens of spies to theMI6 foreign intelligence agency, was given refuge in Britain after he was exchanged in 2010 for Russian spies caught in theWest as part of a Cold War-style spy swap at Vienna airport.

The Kremlin said it was ready to co-operate if Britain askedit for help investigating the incident with Skripal.

Calling it a "tragic situation," Putin's spokespersonDmitryPeskov said the Kremlin had no information about the incident.

Asked to respond to British media speculation that Russiahad poisoned Skripal, Peskov said: "It didn't take them long."

Russia's embassy in London said the incident was being usedto demonize Russia and that it was seriously concerned byBritish media reporting of the Skripal incident.

Russia's foreign spy service, known as the SVR, said it hadno comment to make. Russia'sforeign ministry, and the Russiancounter-intelligence service, the FSB, did not immediatelyrespond to questions submitted by Reuters about the case.

From Moscow to Salisbury

Skripal was arrested in 2004 by Russia's Federal SecurityService (FSB) on suspicion of betraying dozens of Russian agentsto British intelligence. He was sentenced to 13 years in prisonin 2006 after a secret trial.

Skripal, who was shown wearing a track suit in a cage incourt during the sentencing, had admitted betraying agents to MI6 in return for money, some of it paid into a Spanish bankaccount, Russian media said at the time.

But he was pardoned in 2010 by then-President DmitryMedvedev as part of a swap to bring 10 Russian agents held in the United States back to Moscow.

The swap, one of the biggest since the Cold War ended in1991, took place on the tarmac of Vienna airport where a Russianand a U.S. jet parked side by side before the agents wereexchanged.

One of the Russian spies exchanged for Skripal was AnnaChapman. She was one of 10 who tried to blend into Americansociety in an apparent bid to get close to power brokers andlearn secrets. They were arrested by the FBI in 2010.

The returning spies were greeted as heroes in Moscow. Putin,himself a former KGB officer, sang patriotic songs with them.

Skripal, though, was cast as a traitor by Moscow. He isthought to have done serious damage to Russian spy networks inBritain and Europe.

The GRU spy service, created in 1918 under revolutionaryleader Leon Trotsky, is controlled by the military general staff and reports directly to the president. It has spies spreadacross the world.

Since emerging from the John le Carre world of highespionage and betrayal, Skripal lived modestly in Salisbury and kept out of the spotlight until he was found unconscious at 4:15 p.m. local time Sunday.

Wiltshire police said a small number of emergency servicespersonnel were examined immediately after the incident, and allbut one had been released from hospital.

Skripal's wife died shortly after her arrival in Britainfrom cancer, the Guardian newspaper reported. His son died on a recent visit to Russia.

A white and yellow police forensics tent covered the benchwhere Skripal took ill.