Officers have cordoned off a Prezzo restaurant in the city after a man and woman were taken ill in what has been described by police as a “medical incident”.
The intensive care unit at Salisbury District Hospital is on standby to receive the patients, Sky News correspondent Tom Parmenter said.
Roads in the city centre have been cordoned off and Public Health England says it has been informed of the incident.
The restaurant, on the High Street in Salisbury, is a short walk from Queen Elizabeth Gardens, which was closed off after Dawn Sturgess was fatally poisoned by novichok at the end of June.
The same nerve agent was used to target former Russian spy Sergei Skripal and his daughter Yulia, who were found unresponsive on a bench in Salisbury after eating at a Zizzi restaurant about 0.2 miles from Prezzo.
Shortly after police were called to Prezzo on Sunday evening, witnesses reported seeing police “covering someone up” with a blanket, the Salisbury Journal reported.
A witness at a nearby Cafe Rouge restaurant said they saw police bagging something up in the street.
Others said they had been locked in a nearby pub as a hazardous area response team and four ambulances were sent to Prezzo.
Witness Sam Proudfoot, 16, said he saw a man “in a full white body suit with a mask to his mouth going in and out of the back of the ambulance and the restaurant”.
He told Sky News that everyone inside the restaurant had their details taken, and were told to stay within the cordoned area.
Half a dozen police cars, an ambulance, two incident response units and two fire engines were at the scene, he added.
“Police were called by the ambulance service to Prezzo, High Street, Salisbury at 6.45pm today following a medical incident involving two people,” Wiltshire Police said.
“As a precautionary measure, the restaurant and surrounding roads have been cordoned off while officers attend the scene and establish the circumstances surrounding what has led them to fall ill.”