Police have cordoned off part of Salisbury after a man and a woman - reportedly two Russians - were taken ill in an Italian restaurant not far from where Sergei and Yulia Skripal fell unwell after being poisoned with novichok.
A witness claimed the pair affected were showing the same symptoms as the Skripals.
Wiltshire police said they had been called to Prezzo in the city by ambulance crews and that as a precaution, the restaurant and surrounding roads were cordoned off.
They could not confirm that the couple affected were Russian.
Amanda Worne said on Twitter: "Feeling a little apprehensive as Salisbury leaps to action again when 2 Russians are taken ill in Prezzos showing the same symptoms as before ..... AND WE WERE SITTING NEXT TO THEM!
"Now under police supervision waiting for medical team to give us the all clear!"
Earlier, on Facebook, she said they may need blood tests at hospital, adding, "I'm sure we will be fine!"
Prezzo is in the city's High Street, just a few hundred yards from Zizzi, where the Skripals became ill after the nerve agent was put on the front-door handle of his home on 4 March.
Police said in a statement they were called by the ambulance service at 6.45pm tonight "following a medical incident involving two people - a man and a woman".
"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."
South Western Ambulance Service said it dispatched four ambulances including a hazardous area response team after being called at 5.38pm.
The patients were conscious at the last update and were being treated at the scene, a spokeswoman added.
Another witness, Sam Proudfoot, 16, said he saw a person in a hazardous material suit go between the restaurant and the ambulance.
"There's 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 said.
Anyone with information was asked to call police on 101 and quote log number 271.
The Kremlin has repeatedly denied any involvement in the poisoning of the Skripals or Dawn Sturgess and Charlie Rowley in Amesbury, who died after being given a bottle of what she thought was perfume but that contained novichok.
Alexander Petrov and Ruslan Boshirov have been accused by the UK government of attempting to assassinate Sergei Skripal in Salisbury. The two men claim they were in the city as tourists to see the sights.
Zizzi in Salisbury has been permanently closed.