Pub-goers and diners in Salisbury are being urged to wash clothing and possessions after traces of a nerve agent were found.
Public Health England issued the advice as a precaution, but said the risk to the general public remains "low".
Anyone who was in The Mill pub between 1.30pm last Sunday and 11.10pm on Monday, or the nearby Zizzi restaurant between 1.30pm on Sunday and 9pm on Monday, are urged to follow the advice:
- Machine wash clothing worn that day
- Double-bag clothing worn that day which would normally be dry cleaned and await further advice
- Wipe personal effects such as mobile phones, purses and wallets with babywipes
- Hand wash jewellery
Prof Dame Sally Davies, the chief medical officer for England, said the risk of harm was "low".
But she told ITV that after "rigorous scientific analysis" there was some concern that prolonged exposure over weeks and months could cause health problems but it was "not a subject for panic".
She said the advice was a "belt and braces" measure, adding: "The risk to the general public remains low and I am confident none of these customers or staff will have suffered harm."
Trace amounts of the substance used to poison ex-Russian spy Sergei Skripal and his daughter were found at the Mill pub and Zizzi restaurant.
They remain in a critical condition in hospital after being found on a park bench in The Maltings shopping precinct a week ago.
A third victim, Detective Sergeant Nick Bailey, was also admitted to hospital after falling seriously ill when he responded to the emergency, however his condition has since improved.
It comes as Armed Forces personnel continue to assist police.
Soldiers were seen at the South Western Ambulance Service station on Saturday as a vehicle was winched on to the back of an Army low-loader and taken away.
Police investigating the suspected attack have identified more than 240 witnesses and 200 pieces of evidence, Home Secretary Amber Rudd said.