A paramedic who was among the first to respond to Sergei Skripal and his daughter told the Prime Minister they had initially treated the pair for drugs.
The medic was among members of the emergency services and military who met Theresa May at Salisbury's Guildhall, where she thanked them for their response to the nerve agent attack.
The man, named Ian, said he had been in the first ambulance service response car on the scene.
Mrs May asked him: "At that stage you could only treat for what you can see?"
Salisbury MP John Glen interjected to say he had heard initial reports the incident was drug-related.
To which the paramedic replied: "Absolutely that's what I was treating for, that's what we treated them for initially."
Mrs May also met Pc Way and Pc Collins, two Wiltshire Police officers who were first to respond to the emergency call.
Pc Collins told the Prime Minister they had believed the incident was "a routine call".
He said: "It was a routine call, two people on a bench, slumped over, which is nothing out of the ordinary."
Mrs May remarked: "You had no idea what you were dealing with.
"Thank you, what you did was what police do day in and day out.
"A routine call and you don't know what's there.
"You did a great job."
Asked about their colleague Detective Sergeant Nick Bailey, who remains in hospital after suffering exposure to the nerve agent, Pc Collins said: "Obviously our best wishes go to him."