British counter-terrorism officers joined a police investigation on Wednesday after two people were found in critical condition from suspected exposure to an unknown substance near the English city where a former Russian spy was poisoned.

Britain's Sun newspaper reported that the man and woman had been poisoned and were showing similar symptoms to those displayed by ex-double agent Sergei Skripal and his daughter Yulia who were victims of a nerve agent attack in March.

The Sun said samples of the toxin involved had been sent to the nearby military research centre of Porton Down for testing.

The pair, both in their 40s, were hospitalised in a critical condition after being found unconscious on Saturday in Amesbury, seven miles (11 km) north of Salisbury where the Skripals were found slumped unconscious on a bench on March 4.

Britain has blamed the poisoning of the Skripals on Russia, something that Moscow has denied but which caused a serious diplomatic crisis between it and Western states.

"They (couple) are both currently receiving treatment for suspected exposure to an unknown substance," Wiltshire county police, which took the rare step of declaring "a major incident", said in a statement.

"Based upon the number of casualties affected, it is not believed that there is a significant health risk to the wider public."

There was no word on the identity of the couple.

A police spokeswoman declined to confirm whether the couple were believed to have been poisoned or whether Porton Down scientists were now involved. But London police confirmed that specialist counter-terrorism officers were helping the inquiry.

"As Wiltshire police have stated, they are keeping an open mind as to the circumstances surrounding the incident and will update the public as soon and as regularly as possible," a London spokesman said.

Prime Minister Theresa May's spokesman said the government's emergency response committee had met to discuss the incident and that May and ministers were being regularly updated on the developments.

TESTS ON NATURE OF SUBSTANCE

The pair, who were being treated at Salisbury District Hospital, were initially believed to have taken heroin or crack cocaine from a contaminated batch of drugs. More than three days after they were found, tests were still being conducted to ascertain what substance made them ill.

"We are working with the police to understand the circumstances surrounding this incident and it would be inappropriate to make further comment at this time, while we establish how they became unwell," said Cara Charles-Barks, the hospital's chief executive.

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

Police said sites in both Amesbury and Salisbury that they believed the man and woman found in Amesbury had frequented would be cordoned off as a precaution.

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

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

Russia has said it does not possess such nerve agents, did not develop Novichok, and President Vladimir Putin dismissed as nonsense the notion that Moscow would have poisoned Skripal and his 33-year-old daughter.

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

Moscow has hit back by expelling Western diplomats, questioning how Britain knows that Russia was responsible and offering its rival interpretations, including that it amounted to a plot by British secret services.