Italian restaurant Prezzo is currently on lockdown after a man and woman fell ill as Wiltshire Police is working to establish what caused the "medical incident".
Emergency services, including officers in full-body hazmat suits and face masks, four police cars, four ambulances and a fire engine are at the scene.
A source, who was briefed by the emergency services, claimed medics called the police because the symptoms were consistent with Novichok poisoning.
The source said: "The ambulance crew at the site took the decision that the symptoms seemed consistent with Novichok poisoning which is why they called their colleagues in the police.
But he also added: "The symptoms of Novhichok poisoning and particular types of narcotic abuse are very similar."
Police did not mention any link to the previous novichok poisonings and tests are currently being carried out.
Public Heath England has been informed.
Matthew Dean, the leader of Salisbury city council, urged residents not to jump to conclusions and noted there have been a series of false alarms since former Russian military intelligence officer Sergei Skripal and his daughter, Yulia, were poisoned in March.
However, tensions have been building as the restaurant, on the High Street in Salisbury, is only a short walk from Queen Elizabeth Gardens, which was shut down after Dawn Sturgess was fatally poisoned by novichok in June.
She fell ill after supposedly finding a bottle containing the poisonous nerve agent, which was discarded following the Skripal attack.
More to follow...