British police officers stand outside a residential property in Amesbury, England. Picture: AP Photo/Matt Dunham

London - The poison that struck down two Britons has been identified as the same strand of Novichok nerve agent that was used in the attempted murder in March of a former Russian agent and his daughter, Britain's interior minister Sajid Javid said on Thursday.

Javid said it is likely that the pair came into contact with it at a different site from Yulia and her father Sergei Skripal, a former colonel in Russian military intelligence.

He said it has not yet been possible to ascertain whether the nerve agent is from the same batch of Novichok.

"This has been identified as the same nerve agent that contaminated both Yulia and Sergei Skripal," Javid told parliament.

British Prime Minister Theresa May said on Thursday it was deeply disturbing to see two British citizens poisoned by the Novichok nerve agent.

"To see two more people exposed to Novichok in the UK is obviously deeply disturbing and the police, I know, will be leaving no stone unturned in their investigation," May told reporters in Germany where she was meeting Chancellor Angela Merkel.

"My thoughts are with the people of Wiltshire," she added, referring to the county where the victims live and where the latest incident took place.
 
Two British citizens are critically ill after they were exposed to Novichok, the same nerve agent that struck down a former Russian agent and his daughter in March, Britain's top counter-terrorism officer said on Wednesday.

The pair, a local 44-year-old woman and a 45-year-old man, were hospitalised after being found unwell on Saturday in Amesbury, just miles away from Salisbury where ex-double agent Sergei Skripal and his daughter Yulia were attacked in March.

"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.

Britain has accused Russia of poisoning the Skripals with Novichok - a nerve agent developed by the Soviet military during the Cold War - in what is the first known offensive use of such a chemical weapon on European soil since World War Two.

Russia has denied any involvement in their poisoning.