British police say the mysterious illness that befell a former Russian spy and his daughter is the result of a nerve-agent attack.

Metropolitan Police Assistant Commissioner Mark Rowley, who did not specify the exact nature of the substance, announced on Wednesday that the incident was being treated as "attempted murder".

"This is being treated as a major incident involving attempted murder, by administration of a nerve agent," he said.

"Having established that a nerve agent is the cause of the symptoms ... I can also confirm that we believe that the two people who became unwell were targeted specifically."

The police officer who first arrived at the scene of Sunday's events is also said to be in critical condition.

Sergei Skripal, a former colonel in Russia's foreign military intelligence agency, and his 33-year-old daughter, Yulia Skripal, were found unconscious on Sunday near a shopping centre in the city of Salisbury in southern England.

Sally Davies, chief medical officer, said the agent posed little risk to the general public.

A spokesperson for Russia's Ministry of Foreign Affairs dismissed on Tuesday allegations of Russian involvement as "groundless".

"This [Skripal] story will end the same way. The media buzz will be cranked up, there will be groundless allegations without any proof, and then it will all be declared secret, and neither journalists nor society nor politicians and officials will know what really happened," she said.

A public inquiry in 2006 concluded that the Kremlin was likely behind the death of Alexander Litvinenko.

A former spy himself, Litvinenko, died three weeks after drinking green tea laced with radioactive polonium-210 at London’s Millennium Hotel.