Former Russian intelligence officer Sergei Skripal and his daughter were the targets of "attempted murder" using a nerve agent, British police have revealed.

"This is being treated as a major incident involving attempted murder by administration of a nerve agent," Mark Rowley, Britain's top counter-terrorism officer, said.

Skripal, 66, and his daughter Yulia, 33, were found unconscious on a bench outside a shopping centre in the southern city of Salisbury on Sunday afternoon.

"As you know, these two people remain critically ill in hospital," Rowley said.

A police officer who was one of the first to respond to the incident is also in a serious condition in hospital, Rowley said.

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

The case has drawn comparisons to that of Alexander Litvinenko, another Russian former intelligence officer who died in 2006 after drinking tea laced with a radioactive isotope. A British inquiry concluded that Litvinenko was targeted for assassination by the Russian intelligence services.

Former British defence minister Michael Fallon called for a stronger response if Russia was involved in the Skripal affair.

"We've got to respond more effectively than we did last time over Litvinenko. Our response then clearly wasn't strong enough," Fallon told Reuters. "We need to deter Russia from believing they can get away with attacks like this on our streets if it's proved."

Litvinenko's murder sent Britain's ties with Russia to what was then a post-Cold War low. Relations suffered further from Russia's annexation of Crimea and its military backing for Syrian President Bashar al-Assad.

The Skripal case has come shortly before Russia's presidential election on March 18, which Putin is expected to win comfortably, extending his rule by a further six years.

The former KGB officer has been president since 2000.