Britains counter terrorism police will lead the investigation into a mysterious illness that struck down a Russian former double agent and his daughter in the southern English city of Salisbury, police said on Tuesday. Sergei Skripal, 66, and his 33-year-old daughter Yulia were critically ill in hospital after being found unconscious on Sunday on a bench outside a shopping center in the city. "Due to the unusual circumstances, it has been decided that the Counter-Terrorism Policing network will lead the investigation as it has the specialist expertise to do so," Londons Metropolitan Police said in a statement. "It has not been declared a terrorist incident and at this stage we are keeping an open mind as to what happened." Russia insists it has "no information" on what could have led to the incident, but says it is open to co-operate in the police investigation if requested. Former agent Mr Skripal, whose wife, son and older brother have all died in the past two years, was granted refuge in the UK following a "spy swap" in 2010. Calling it a “tragic situation,” Putin’s spokesman Dmitry Peskov said the Kremlin had no information about the incident. Asked to respond to British media speculation that Russia had poisoned Skripal, Peskov said: “It didn’t take them long.” Russia’s embassy in London said the incident was being used to demonize Russia and that it was seriously concerned by British media reporting of the Skripal incident. Police are currently examining CCTV footage, filmed by a Salisbury gym, showing an unidentified man and woman walking near to the location where Mr. Skripal and his daughter were found. Foreign Secretary Boris Johnson threatened on Tuesday to pull out of the soccer World Cup in Russia if Moscow was shown to be behind the mysterious illness that struck down Skripal. “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 the British parliament. “It is clear that Russia, I’m afraid, is now in many respects a malign and disruptive force, and the UK is in the lead across the world in trying to counteract that activity.” If Moscow was shown to be behind Skripal’s illness, Johnson said, it would be difficult to see how Britain could attend the World Cup in Russia in June and July. England is the only British national team to have qualified for the tournament, which is held every four years. A previous British inquiry said President Vladimir Putin probably approved the 2006 murder of ex-KGB agent Alexander Litvinenko with radioactive polonium-210 in London. The Kremlin has repeatedly denied any involvement in Litvinenko’s killing. Litvinenko, 43, an outspoken critic of Putin who fled Russia for Britain six years before he was poisoned, died after drinking green tea laced with the rare and very potent radioactive isotope at London’s Millennium Hotel. It took some time for British doctors to discern the cause of Litvinenko’s illness. His murder sent Britain’s relations with Russia to what was then a post-Cold War low. Relations suffered further from Russia’s annexation of Crimea and military backing of head of Syrian regime Bashar al-Assad against rebels trying to topple him.FROM MOSCOW TO SALISBURY Skripal was arrested in 2004 by Russia’s Federal Security Service (FSB) on suspicion of betraying dozens of Russian agents to British intelligence. He was sentenced to 13 years in prison in 2006 after a secret trial. Skripal, who was shown wearing a track suit in a cage in court during the sentencing, had admitted betraying agents to MI6 in return for money, some of it paid into a Spanish bank account, Russian media said at the time. But he was pardoned in 2010 by then-President Dmitry Medvedev 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 in 1991, took place on the tarmac of Vienna airport where a Russian and a US jet parked side by side before the agents were exchanged. One of the Russian spies exchanged for Skripal was Anna Chapman. She was one of 10 who tried to blend into American society in an apparent bid to get close to power brokers and learn 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.