Police believe a nerve agent was used to deliberately poison a former Russian double agent and his daughter, Britain's top counter-terrorism officer says, in a case that threatens to further damage London's ties with Moscow.

Key points: The case of Skripal and his daughter is being treated as attempted murder

The case of Skripal and his daughter is being treated as attempted murder Police declined to give exact details of the poison but confirmed it was a nerve gas

Police declined to give exact details of the poison but confirmed it was a nerve gas Skripal was convicted in Russia in 2006 of spying for Britain

Sergei Skripal, once a colonel in Russia's GRU military intelligence service, and his 33-year-old daughter, Yulia, were found slumped unconscious on a bench outside a shopping centre in the southern English city of Salisbury on Sunday afternoon.

Both remain critically ill and a police officer who attended the scene is also in a serious condition in hospital.

"This is being treated as a major incident involving attempted murder by administration of a nerve agent," Metropolitan Police counterterrorism chief Mark Rowley said in a statement.

He declined to give specific details of the substance used or how the victims were exposed to the poison, but said police believed they were specifically targeted.

The police statement came as Russian President Vladimir Putin warned the West to stop trying to contain Russia.

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"Those who serve us with poison will eventually swallow it and poison themselves," Mr Putin said in comments during a series of interviews for a documentary released on Wednesday, unrelated to the Skripal case.

"We are a great power, and no one likes competition."

England's chief medical officer said the incident posed a low risk to the wider public.

Sergei Skripal, who was a Russian agent, and his daughter Yulia Skripal. ( AP: Misha Japaridze, Facebook: Yulia Skripal )

While Mr Rowley would not say any more about the investigation, a US security source, speaking on condition of anonymity, said the main line of police inquiry was that Russians may have used the substance against Skripal in revenge for his treachery.

Skripal betrayed dozens of Russian agents in dealing with British intelligence before his arrest by Russian authorities in 2004.

He was sentenced to 13 years in prison in 2006 after a secret trial and in 2010 was given refuge in Britain after being exchanged for Russian spies caught in the West as part of a Cold War-style spy swap at Vienna airport.

Sorry, this video has expired An eyewitness gives a graphic account of the poisoned couple

On Tuesday, British Foreign Secretary Boris Johnson said if Moscow was behind the incident then Britain could look again at sanctions and take other measures to punish Russia, which he cast as a "malign and disruptive" state.

Russia denied any involvement, scolded Mr Johnson for "wild" comments and said anti-Russian hysteria was being whipped up intentionally to damage relations with London.

"It's very hard not to assess this [speculation] as provocative black PR designed to complicate relations between our two countries," Foreign Ministry spokeswoman Maria Zakharova told reporters in Moscow.

Police search restaurant, pub where two had been

Investigators work inside the Zizzi restaurant where Sergei Skripal and his daughter dined. ( AP: Steve Parsons/PA )

Police said new cordons had been added near Solstice Park, a business park, in the town of Amesbury near to Salisbury.

They have sealed off the area of Salisbury where Skripal was found as well as the Zizzi pizza restaurant where they dined and the Bishop's Mill pub where they had a drink.

The British capital has been dubbed "Londongrad" due to the large quantities of Russian money which have poured in since the 1991 fall of the Soviet Union.

It is the Western city of choice for many oligarchs from the former Soviet Union.

Britain has specifically drawn parallels with the 2006 murder of ex-KGB agent Alexander Litvinenko, who was killed with radioactive polonium-210 in London.

A previous British inquiry said Mr Putin probably approved the murder of Litvinenko, who died after drinking green tea laced with the rare and very potent radioactive isotope at London's Millennium Hotel.

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AP/Reuters