Hundreds of diners and pub-goers have been urged to wash clothes and other items a week after potentially coming into contact with the nerve agent used to attack former spy Sergei Skripal and his daughter.
The "precautionary advice" was issued after traces of the substance were found in The Mill pub and the nearby Zizzi restaurant, in Salisbury.
England's chief medical officer Dame Sally Davies said she believed fewer than 500 people would have been in either venue
between Sunday lunchtime and Monday night.
Dr Jenny Harries, joint director of Public Health England (PHE), defended the decision to issue new guidance seven days after Mr Skripal, 66, and his daughter, Yulia, 33, were found on a bench in the Wiltshire city. They remain in Salisbury District Hospital in a critical, but stable condition.
She told reporters at a press conference on Sunday: "This is about a very, very small risk of repetitive contact with traces of contamination that people may have taken out. The advice we're giving today about washing clothes - very simple things... that will remove that risk as we go forward."
PHE advised people who visited The Mill pub between 1.30pm last Sunday and closing time at 11.10pm on Monday, and Zizzi
restaurant between 1.30pm Sunday and 9pm the next day - should take action.
This includes advice to:
- 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 baby wipes
- Hand wash jewellery
Dame Davies said: "I want to reassure the general public that the risk to us all from this incident in Salisbury has not changed, and that the risk to us all remains low.
"Rigorous scientific analysis continues, but we have now learnt that there has been some trace contamination by the nerve
agent in both The Mill pub and Zizzi's restaurant in Salisbury.
"I am confident this has not harmed the health of anyone who was in The Mill pub or Zizzi's restaurant.
"However, some people are concerned that prolonged long-term exposure to these substances may, over weeks and particularly months, give rise to health problems.
"I'm therefore advising - as a belt and braces approach - that people who were in (Zizzi's or the Mill during the specified times) ... should clean the clothes they wore and the possessions they handled while there.
"This is precautionary advice aimed at only those people who were at the venues at these times, which I believe to be below 500 people. I am confident none of these customers will have suffered harm."
It comes as Detective Sergeant Nick Bailey, remains in hospital in a serious but stable condition in hospital, where he
is said to be conscious and talking.
Temporary Chief Constable Kier Pritchard of Wiltshire Police said the "unprecedented incident", involving one of his officers, "sent shockwaves through the force".
He was "unable to clarify how long" crime scenes at a number of sites around Salisbury, will remain in place.
Meanwhile, armed forces personnel continue to assist police
Soldiers removed two ambulances from the South Western Ambulance Service station on Saturday, while more soldiers were seen at Bourne Hill Police Station.
Police investigating the suspected attack have identified more than 240 witnesses and 200 pieces of evidence, Home Secretary Amber Rudd said.
The attack on Mr Skripal and his daughter was a "violation" of the city, the Bishop of Salisbury said.
The Rt Rev Nicholas Holtam told BBC Radio Solent: "There has been a violation of this place and this community and it is more than an attack on individuals."
Cordons remain in place across the city as investigators in hazmat clothing continue to delicately scour for clues.
Wiltshire Police confirmed it has charged a man who breached one of the cordons on Friday evening.
Jamie Knight, 30, from Salisbury, is suspected of assaulting a police officer, common assault, criminal damage to a police vehicle and a racially aggravated public order offence.
He will appear before magistrates in Swindon on Monday.