Britain has threatened to pull out of the soccer World Cup in Russia if Moscow is shown to be behind the mysterious illness that struck down a Russian former double agent convicted of betraying dozens of spies to British intelligence.

Foreign Secretary Boris Johnson named Sergei Skripal, a former colonel in Russia’s GRU military intelligence service, and his daughter Yulia as the two people who were found unconscious on Sunday on a bench outside a shopping centre in southern England.

Mr Skripal, 66, and his 33-year-old daughter were exposed to what police say is an unknown substance in the English city of Salisbury. 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,” Mr Johnson told the British parliament on Tuesday.

Sergei Skripal was convicted of spying by Russia in 2006. Photo: Getty

“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 Mr Skripal’s illness, Mr 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.

Yulia Skripal is also in a critical condition. Photo: Facebook

The Kremlin has repeatedly denied any involvement in Mr Litvinenko’s killing.

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

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

Mr Skripal, who passed the identity of dozens of spies to the MI6 foreign intelligence agency, was given refuge in Britain after he was exchanged in 2010 for Russian spies caught in the West as part of a Cold War-style spy swap at Vienna airport.

A UK inquiry blamed Russia for the killing of former agent Mr Litvinenko. Photo: Getty

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

Calling it a “tragic situation”, Mr Putin’s spokesman Dmitry Peskov said the Kremlin had no information about the incident.

Russia’s embassy in London said the incident was being used to demonise Russia and that it was seriously concerned by British media reporting of the Skripal incident.

-Reuters