Amanda Worne told Sky News that she was in Prezzo in Salisbury, Wiltshire, sitting next to a “glamorous blonde” woman who seemed “agitated” and then became extremely upset after finding a man collapsed in the restaurant’s toilet.
Ms Worne said: “We were told by a police officer that they were two Russians showing symptoms of novichok and that we would probably have to go to hospital for tests.”
The man, 42, and woman, 30, became unwell at the restaurant on Sunday evening, prompting police to declare a major incident. The nationality of the pair have not been confirmed by police.
Wiltshire Police said they were assessed at Salisbury District Hospital due to concerns they had been “exposed to an unknown substance”.
Tests are continuing and the cordon will remain at the restaurant “as a precautionary measure”, officers said on Monday afternoon.
The restaurant, on Salisbury’s high street, is a short walk from Zizzi, where former Russian spy Sergei Skripal and his daughter Yulia ate before they were found slumped on a bench in the Wiltshire city in March.
The two spent months recovering in hospital from that attack which Theresa May, backed by several Western countries, was carried out by Russian agents with nerve agent novichok.
Russia continues to deny the accusations.
Prezzo is also close to Queen Elizabeth Gardens, which was closed off after Dawn Sturgess was fatally poisoned by novichok at the end of June.
Ms Worne said the diners realised they were possibly caught up in another nerve agent attack.
“You are just absolutely gobsmacked,” she said.
“You can’t believe it could be happening again.
“Then 20 minutes after that, public health officials arrived and they said they didn’t know what it was but it wasn’t novichok. I asked ‘do I have to burn my jacket?’
“They said ‘no you can go’.”
A cordon remains at the restaurant, although novichok was not involved in the incident, police have confirmed.
Officers added that “recent events in the city and concerns that the pair had been exposed to an unknown substance” had prompted a “highly precautionary approach”.
Detective Sergeant Jonathan Davies-Bateman said: “Although we were able to rule out the presence of novichok quickly, tests are ongoing to understand what, if anything, the pair have come into contact with which may have caused them to fall unwell.
“It is crucial that this is done methodically.
“At this stage, it is unclear as to whether or not a crime has been committed.”