Zizzi restaurant in Salisbury has been closed "as a precaution" in connection with the major incident involving an ex-Russian spy.
Sergei Skripal, 66, and an as-yet unidentified woman said to be in her thirties, were found unconscious on a bench at the The Maltings shopping centre.
They did not have any visible injuries and are currently in critical condition after they were exposed to an "unknown substance".
Wiltshire Police said: "A restaurant in Salisbury City Centre has been closed in connection with the on-going major incident.
"Zizzi, on Castle Street, has been secured as a precaution by Wiltshire Police while the investigation continues.
"Public Health England are aware of this and have reiterated that, based on the evidence to date, there is no known risk to the public's health."
The case has drawn immediate comparison to the murder of former FSB officer Alexander Litvinenko which used radioactive polonium-210 believed to have been administered in a cup of tea.
Investigators are understood to be reviewing the death of Litvinenkno to see if there are any similarities between the modus operandi of that case and this incident.
“They are currently being treated for suspected exposure to an unknown substance,” said temporary Assistant Chief Constable Craig Holden. “Both are currently in a critical condition in intensive care.
“Because we are still at the very early stages of the investigation, we are unable to ascertain whether or not a crime has taken place. A major incident has been declared today and a multi-agency response has been coordinated.”
