Britain warned Russia of a robust response if the Kremlin was behind a mysterious illness that has struck down a former double agent convicted of betraying dozens of spies to British intelligence.

Foreign Secretary Boris Johnson named Sergei Skripal, once a 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.

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

"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 on Tuesday.

"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."

Prime Minister Theresa May was briefed at a meeting of the National Security Council on the investigation into the incident, her spokesman said without elaborating.

If Moscow were shown to be behind Skripal's illness, Johnson said, it would be difficult to see how UK representation could go to the World Cup in Russia in a normal way. A government source said that meant attendance of ministers or dignitaries.

Russian foreign ministry spokeswoman Maria Zakharova said Johnson's comments were "wild".
British authorities said there was no known risk to the public from the unidentified substance, but they sealed off the area where Skripal was found, which included a pizza restaurant and a pub, in the centre of Salisbury.

Counter-terrorism police are now leading the investigation though they said they believe there no risk to the public. Samples from the scene are being tested at Porton Down, Britain's military research laboratory, the BBC said.

Wiltshire police said a small number of emergency services personnel were examined immediately after the incident and all but one had been released from hospital.

A white and yellow police forensics tent covered the bench where Skripal was taken ill.

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

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," Putin's spokesman Dmitry Peskov said the Kremlin had no information about the incident.

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