Two British citizens are critically ill after they were poisoned with the Novichok nerve agent that struck down a former Russian spy and his daughter earlier this year.

The British pair, a 44-year-old woman and a 45-year-old man, were hospitalised after being found unwell on Saturday in Amesbury, just 11km north of Salisbury where ex-double agent Sergei Skripal and his daughter Yulia were found slumped unconscious on a bench on March 4.

“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 on Thursday morning (AEST).

UK counter-terrorism police are now leading the investigation though Mr Basu said it was unclear how the two people came into contact with the nerve agent or whether they had been specifically targeted.

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

Sam Hobson, a friend of two people who were found unconscious, talks to media outside Amesbury Baptist Centre. Photo: Getty

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

He told Sky News she was “having a fit, foam coming out of her mouth”.

Mr Rowley collapsed later the same day.

“He was sweating loads, dribbling … he was rocking backwards and forwards,” Mr Hobson said.

“There was no response from him. He didn’t even know I was there.”

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.

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

The attack prompted the biggest Western expulsion of Russian diplomats since the Cold War as allies in Europe and the US 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.

Ms May’s spokesman said the government’s emergency response committee had met twice on Wednesday to discuss the incident.

Paramedics were called on Saturday morning to a house in Amesbury after the woman collapsed and returned later in the day when the man also fell ill.

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 inquiry for us.”

The hospital is where the Skripals also spent weeks in a critical condition before slowly recovering and being discharged.

-AAP