Up to 500 Salisbury pub-goers and diners have been told to wash possessions as a precaution after nerve agent traces were found.
Trace amounts of the substance used to poison ex-Russian spy Sergei Skripal and his daughter were found on and around a table where the pair ate at Zizzi restaurant, the BBC has learned.
The table, along with other items, has been removed and destroyed.
Scientists have advised police it could take weeks for the premises to reopen.
Traces of the nerve agent were also found at the Mill pub in Salisbury.
Sergei and Yulia Skripal, who remain in a critical but stable condition in hospital, are understood to have eaten on a table away from other diners.
Prof Dame Sally Davies, the chief medical officer for England, said the risk of harm to fellow diners was "low".
The advice to wash possessions applies to anyone in either venue between 13:30 GMT on Sunday 4 March and closure on Monday:
- Clothes should be washed, ideally in a washing machine
- Clothes which cannot be washed, for example if they need dry cleaning, should be double bagged in plastic until further notice
- Mobile phones, handbags and other electronic items should be wiped with baby wipes, which should be bagged in plastic and put in the bin
- Other items such as jewellery and glasses should be washed with warm water and detergent
- Hands should be washed after the handling of any items suspected of being contaminated.
Dame Sally said 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: "I am confident none of these customers or staff will have suffered harm."
But people in Salisbury are concerned the advice was not given sooner.
Steve Cooper, who was at the Mill pub with his wife and dog for a couple of hours last Sunday afternoon, told the BBC he was outraged.
He said he would now wash his shoes, watch and phone but feared using baby wipes would not get rid of a nerve agent.
Some of his friends, who had been in the pub at the same time and seen Mr Skripal head to the toilet, could not remember what they had been wearing that day, he added.
Mr Cooper said he had become increasingly concerned for his safety in the last few days.
"I'd like to know what are the long-term implications to me and my wife," he said.
Dame Sally said it had been a "painstaking process" and the "scientific tests take time" but no harm had been caused by the wait.
Alastair Hay, professor emeritus of environmental toxicology at the University of Leeds, said nerve agents degrade in the environment.
"Contact with moisture will lead to breakdown of the nerve agent - this is why people having visited the restaurant or pub in question last Sunday afternoon or Monday are being advised to wash their possessions," he said.
Mr Skripal, 66, and his daughter Yulia, 33, were found slumped on a bench in the city on Sunday.
Det Sgt Nick Bailey, who fell ill attending the pair, remains seriously ill in hospital but has been talking to his family.
The pub and restaurant are two of five sites in Salisbury focused on by investigators.
Mr Skripal's home and the cemetery where Mr Skripal's wife and son are buried are also being examined.
At a press conference on Sunday, Chief Constable Kier Pritchard of Wiltshire Police said he was "unable to clarify" how long those crime scenes would remain closed to the public.
Members of the military are assisting police for a third day having previously helped with the removal of vehicles of interest including an ambulance.
More than 250 counter terrorism police are now involved in the investigation, which has yielded 200 pieces of evidence so far and more than 240 witnesses.
At a Sunday service at Salisbury Cathedral, the bishop - the Right Reverend Nicholas Holtam - said as well as a "shocking attack on two individuals", the attack was a "violation of our community".
Mr Skripal, a retired Russian military intelligence officer, was convicted by the Russian government of passing secrets to MI6 in 2004, but given refuge in the UK in 2010 as part of a "spy swap".
Russia has denied any involvement.
Do you have any information to share on this story? Email firstname.lastname@example.org.
Please include a contact number if you are willing to speak to a BBC journalist. You can also contact us in the following ways: