Traces of the nerve agent used against former Russian spy Sergei Skripal and his daughter, Yulia, have been found at the restaurant where they ate on Sunday afternoon, the BBC understands.
The substance was found in one part of Zizzi in Salisbury during a continuing forensic examination.
The pair were found two hours after finishing their meal collapsed on a park bench. Both are critically ill.
No-one who was in the restaurant at the same time is thought to be in danger.
There is also no suggestion that anyone dining at the time had anything to do with the nerve agent.
The restaurant is currently surrounded by a large screen while an investigation continues inside.
Zizzi is one of five sites in the small Wiltshire city of Salisbury at the heart of the operation.
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.
Mr Skripal, 66, was convicted by the Russian government of passing secrets to MI6, but given refuge in the UK in 2010 as part of a "spy swap".
Russia has denied any involvement. UK Foreign Secretary Boris Johnson has said the country will respond "robustly" if Moscow is found to have been behind the incident.
Salisbury's London Road cemetery was cordoned off on Friday, but police confirmed officers were not exhuming a body.