SALISBURY, ENGLAND (REUTERS, AGENCE FRANCE-PRESSE) - Britain warned it would respond robustly, including pulling out of the World Cup, if Russia was shown to be behind the mysterious illness that struck down a former Russian double agent in England.

Foreign Secretary Boris Johnson named Mr Sergei Skripal, a former colonel in Russia's GRU military intelligence service convicted of betraying dozens of spies to British intelligence, 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 said was an unknown substance in the city of Salisbury. Both are still critically ill in intensive care. An emergency service worker also remains in hospital.

Interior Minister Amber Rudd said on Wednesday (March 7) the British authorities know more about the substance which Nr Skripal was exposed to before he became critically ill, , adding that police would make a further statement later.

"We do know more about the substance and the police will be making a further statement this afternoon in order to share some of that," Ms Rudd told reporters after chairing a meeting of top ministers to discuss the Skripal incident.

Britain's counter-terrorism police will lead the investigation, police said yesterday.

"We don't know exactly what has taken place in Salisbury, but if it is 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 Parliament yesterday.

"It is clear that Russia, I am 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.

He also raised the prospect of more sanctions. "Should evidence emerge that shows state responsibility, then Her Majesty's government will respond appropriately and robustly," he said.

"It may very well be that we are forced to look again at our sanctions regime and other measures that we may seek to put in place."

A previous British inquiry said Russian President Vladimir Putin probably approved the 2006 murder of former KGB agent Alexander Litvinenko with radioactive polonium in London. The Kremlin has repeatedly denied any involvement.

Mr Litvinenko, 43, an outspoken critic of Mr 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.

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 Syrian President Bashar al-Assad against rebels trying to topple him.

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

Mr Skripal, who passed the identities of dozens of spies to the MI6 foreign intelligence agency, was arrested in 2004 by Russia's Federal Security Service on suspicion of betraying dozens of Russian agents to British intelligence. He was sentenced to 13 years in prison in 2006.

He was given refuge in Britain after he was exchanged in 2010 for Russian spies caught in the United States as part of a Cold War-style spy swop at a Vienna airport.

The Kremlin said yesterday it was ready to cooperate if Britain asked it for help investigating the incident with Mr 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.

Mr Skripal has lived modestly in Salisbury and kept out of the spotlight until he was found unconscious on Sunday. The Sun newspaper said Mr Skripal's wife was killed in a car accident shortly after her arrival in Britain. His son was killed in a car accident in Russia.

Other high-profile cases involving Kremlin opponents

ALEXANDER LITVINENKO

Mr Alexander Litvinenko, a former Russian spy who fled to Britain, died in 2006 after being exposed to polonium, a rare and highly radioactive metal.

The 43-year-old had drunk green tea laced with the poison at London's Millennium Hotel, where he had been living in exile.

He was known for criticising Russian President Vladimir Putin of being corrupt.

A British inquiry in January 2016 found that Mr Putin had "probably approved" of the killing of Mr Litvinenko.

ROMAN TSEPOV

Russian businessman Roman Tsepov, 42, died after drinking a cup of tea at a Federal Security Service (Russia's successor agency to the KGB) office in St Petersburg on Sept 11, 2004.

Mr Tsepov was a one-time associate of Mr Putin and had previously been arrested for illegal storage of weapons and drugs.

A post mortem found that he had been poisoned by an unnamed radioactive material.

BORIS NEMTSOV

Russian opposition leader Boris Nemtsov, one of Mr Putin's most vocal critics, was shot dead on Feb 27, 2015, as he walked across a bridge near the Kremlin.

Mr Nemtsov, 55, had been working on a report examining Russia's role in the conflict in Ukraine.

Five Chechen men were convicted of his killing last year, but Mr Nemtsov's family believes his real killer remains at large, and that he was assassinated for his political work.

ANNA POLITKOVSKAYA

Russian investigative journalist Anna Politkovskaya was shot in her Moscow apartment building in 2006.

Ms Politkovskaya, 48, had chronicled corruption under Mr Putin and human rights abuses during Russia's conflict with separatists in Chechnya.

There were suspicions that Mr Putin was involved, but five men, including four members of the same Chechen family, were eventually found guilty of the murder.

However, Ms Politkovskaya's family and former colleagues remain convinced that the masterminds have not been brought to justice.

STANISLAV MARKELOV AND ANASTASIA BABUROVA

Human rights lawyer Stanislav Markelov, 34, was gunned down in the middle of a Moscow street after leaving a press conference in 2009. Opposition journalist Anastasia Baburova, 25, who was with him, was also shot dead.

Mr Markelov, who had represented victims of the 2002 Moscow theatre siege, where more than 100 hostages were killed by Russian special forces, was assumed to be the main target.

He also worked for opposition newspaper Novaya Gazeta and had dealt with the murder case of the paper's correspondent, Ms Politkovskaya.

Ng Huiwen