Britain's top counter-terrorism officer has confirmed two British citizens who are critically ill in hospital were poisoned with Novichok, the same nerve agent that struck down a former Russian agent in March.

Key points: No intelligence to suggest the couple were deliberately targeted

No intelligence to suggest the couple were deliberately targeted Police investigating whether case is linked to poisoning of former Russian spy

Police investigating whether case is linked to poisoning of former Russian spy The man and woman remain in critical conditions in hospital

Police said there was no evidence they had visited the same sites that were decontaminated after the Skripal case.

"I have received test results from Porton Down [military research centre] which show that the two people have been exposed to the nerve agent Novichok," Neil Basu, Britain's most senior counter-terrorism officer, told reporters.

UK counter-terrorism police are now leading the investigation and said the possibility that the case is linked with the poisoning of former Russian spy Sergei Skripal and his daughter Yulia was a line of enquiry.

But Mr Basu said there was no intelligence to indicate that the man and woman had been deliberately targeted.

He also told reporters police did not yet know how the Novichok nerve agent was transmitted.

Health chiefs said the risk to the public was low, though the exposure will likely stoke fears that traces of the nerve agent remain in the area.

"As the country's chief medical officer, I want to reassure the public that the risk to the general public remains low," England's chief medical officer Sally Davies told reporters.

Charlie Rowley, left, and Dawn Sturgess fell ill within hours of each other. ( Facebook )

British Prime Minister Theresa May's spokesman said the Government's emergency response committee had met to discuss the incident.

Home Secretary Sajid Javid will chair a meeting of the emergency response committee on Thursday.

"The working theory is currently that this exposure was accidental, rather than a second attack along the lines of that on Sergei and Yulia Skripal in Salisbury earlier this year," Mr Javid said.

Sorry, this video has expired One of the nerve agent victims is tended to by emergency authorities

The man, 45, and woman, 44, remain in a critical condition in hospital after being found unconscious at a residential building in Amesbury on Saturday evening (local time).

Amesbury is just 13 kilometres north of Salisbury, where the Skripals were found slumped unconscious on a bench on March 4.

Police have not named the couple, but friends have named them as Charlie Rowley and Dawn Sturgess.

'Foam coming out of her mouth'

Sorry, this video has expired 'He was just sweating, dribbling, making weird noises': Friend of the victims describes watching the substance take hold

Sam Hobson said he was with them on Saturday, when Ms Sturgess fell ill first.

"She was on the floor having a fit and foaming at the mouth," Mr Hobson said.

Mr Rowley collapsed later the same day.

"He was rocking against the wall making funny noises and his eyes were wide open, glazed, red and pinpricked and he was just sweating, dribbling, making weird noises," Mr Hobson said.

"I was speaking to him and was getting no response."

A man and a woman were found unconscious in a residential building on Saturday evening. ( AP: Matt Dunham )

The pair, who are being treated at Salisbury District Hospital, were initially believed to have taken heroin or crack cocaine from a contaminated batch but tests showed they had been poisoned with Novichok, a nerve agent developed by the Soviet military in the Cold War.

"We are not in a position to say whether the nerve agent was from the same batch that the Skripals were exposed to," Mr Basu said.

"The possibility that these two investigations might be linked is clearly a line of enquiry for us."

Police have cordoned off at least five different areas, including a park and a property in Salisbury, and a pharmacy and a Baptist church community centre in Amesbury, although health chiefs said the risk to the public was low.

Salisbury and surrounding towns have only recently begun to recover from the frightening weeks at the centre of an international spy drama.

Police from 40 departments in England and Wales returned home in June after months working on the Skripal case, and specially trained workers have spent months decontaminating sites around the city.

The British Government has pledged 2.5 million pounds ($4.5 million) to local businesses to make up for lost revenue in the area, which is a gateway to Stonehenge, the ancient stone circle that is a huge tourist destination.

Britain accused Russia of poisoning Mr Skripal with the Novichok nerve agent, the first known offensive use of such a chemical weapon on European soil since World War II.

The attack prompted the biggest Western expulsion of Russian diplomats since the Cold War as allies sided with Prime Minister Theresa May's view that Moscow was either responsible or had lost control of the nerve agent.

Russia denied any involvement and suggested Britain had carried out the attack to stoke anti-Russian hysteria.

A playing field near the Amesbury Baptist Church is cordoned off. ( Reuters: Henry Nicholls )

Reuters/AP