Yulia Skripal, 33, had just finished eating at Zizzi, a pizza restaurant in the English city of Salisbury with her 66-year-old father, Sergei Skripal.

After leaving, the pair was seen walking through an alley near the restaurant. Minutes later, they were found unconscious on a bench outside a shopping centre.

Witness Freya Church, 27, was leaving her job at Snap Fitness on Sunday about 4pm, when she saw Yulia and her father on the bench.

“I was particularly worried about her,” Freya told The Telegraph.

“She was slumped over on the man’s shoulder. To be honest, I thought they might be homeless but they were perhaps better dressed.

“I just thought this is weird, especially as she was clearly quite a bit younger than him.

“There was no one else there near them at this point. No one was helping them.”

British police and counter-terrorism officers believe the pair have been poisoned, and are currently racing to determine the substance responsible for their illness.

It's been revealed that Sergei - who, along with his daughter, is critically ill in intensive care - is a former Russian double agent convicted of treason in Moscow for giving the identity of dozens of spies to British intelligence.

Sergei - described by his neighbours as a quiet man who dressed casually and drove a BMW - 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'.

In relation to Sergei and his daughter's illness, British authorities said there was no known risk to the public. As a precaution, police sealed off the area where the former spy was found and the pizza restaurant where the pair had dined. Some investigators wore yellow chemical suits.

"We're speaking to witnesses, we're taking forensic samples at the scene, we're doing toxicology work and that will help us to get to an answer," Britain's top counter-terrorism officer, Mark Rowley, told BBC radio.

"We have to remember: Russian exiles aren't immortal, they do all die and there can be a tendency to conspiracy theories. But likewise we have to be alive to the fact of state threats."

According to Yulia's Facebook page, she is currently employed by PepsiCo in Moscow, and had previously worked at the Holiday Inn in Southampton.

A picture of the cathedral in Salisbury, where her father lived, is also featured on her Facebook page.

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

Foreign Secretary Boris Johnson said England would be willing to pull out of the upcoming 2018 FIFA World Cup if investigations proved Moscow was behind Sergei and Yulia's 'poisoning'.

"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."

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.

With AAP.