There is "nothing to suggest" that Novichok caused two people to fall ill in the British town where a Russian ex-spy and his daughter were poisoned by the deadly nerve agent, police said today.

Parts of Salisbury in southwest England were sealed off and a hazardous area response team was called to the scene after the pair became unwell at an Italian restaurant on Sunday evening.

"Due to recent events in the city and concerns that the pair had been exposed to an unknown substance, a highly precautionary approach was taken by all emergency services," Wiltshire Police said in a statement.

"We can now confirm that there is nothing to suggest that Novichok is the substance."

Police said the couple, a man in his 40s and a woman in her 30s, were assessed in hospital where they remained under observation.

It was unclear if any crime had been committed, police said, adding that the events were no longer being treated as a major incident but enquiries were ongoing.

It was in Salisbury in March that Russian double agent Sergei Skripal and his daughter Yulia were poisoned with Novichok, a powerful nerve agent created in the Soviet Union.

Britain has accused Russia of orchestrating that attack, a charge Moscow has denied.

Skripal and his daughter survived, but a British couple, Dawn Sturgess and Charlie Rowley, came into contact with the same nerve agent in a town near Salisbury months later and Sturgess, 44, died in hospital.