Police in hazmat suits closed the road in Salisbury where Sergei Skripal and his daughter Yulia were poisoned with nerve agent Novichok after a man in his thirties fell ill.
The fears of a similar incident proved to be a false alarm and police have said they have no concerns about the man's health.
Emergency services were pictured outside the restaurant where the Skripals dined before the poisoning.
Witnesses reported commotion near today's incident, which follows the poisoning of two others with the nerve agent in Amesbury.
People have shared images of police at the scene, with one writing online: 'Major activity in #Salisbury. Police and search and rescue running into the railway station. Sirens audible for ages. Road shut in city centre, tent being erected here.'
Another wrote: 'Looks like the men in green suits are responding to a person outside Tesco's in the Salisbury city centre next to Zizzi's restaurant a lot of shouting, Screaming at the individual on the floor.'
Wiltshire Police say there is nothing to suggest any wider risk to the public at this time but people are being asked to avoid the area for the time being, Sky News reports.
It comes after Charlie Rowley and his partner Dawn Sturgess were poisoned after relaxing in a park near the Zizzi. Ms Sturgess, 44, died as a result and Mr Rowley remains in intensive care.
They apparently handled an item discarded following the attack on the Skripals in Salisbury on March 4.
The Kremlin denies accusations that it tried to assassinate Mr Skripal with Novichok and that it is responsible for the death on Sunday of Mrs Sturgess, a mother of three.
What are the symptoms of Novichok and how does it linger?
Police have confirmed that a man and a woman from Amesbury, who are in a critical condition, have been exposed to the nerve agent Novichok.
It is the same substance used in the attack and poisoning of former Russian spy Sergei Skripal and his daughter Yulia in March.
What is Novichok?
A group of nerve agents which are more potent and lethal than VX or sarin.
They are made of two separate non-toxic substances that work as a nerve agent when brought together.
Dr Andrea Sella, professor of inorganic chemistry at University College London, said: "Novichok is not really very different from all the classics, you've got the same basic chemical framework at the heart of it.
"I'm not sure it's ever really been used. There's not much experience of seeing these things, they would have recognised it was some sort of nerve agent, which is part of the reason for the delay [in identifying it]."
How long can it linger?
Dr Sella said it is "very disturbing" that the agent has been found four months after the first attack, but Novichok is designed not to break down.
"These things are designed to be persistent," he said.
"They don't evaporate, they don't break up in water. The last four months have been dry so I suspect they can be there for quite a long time."
If the substance was sealed, perhaps in a drinks bottle, then it could take even longer to break down, he added.
Why was it created?
Novichok, which means newcomer in Russian, was developed by the Soviet Union in the 1970s as a new kind of chemical weapon that would be harder to detect, more potent than existing nerve agents and exempt from the Chemical Weapons Treaty.
Dr Sella said: "Novichok agents are ones that were kept very quiet by the Russians and developed to try and gain advantage against the more conventional things they knew Western governments had."
How does it work?
Novichok and other nerve agents attack the nervous system and stop chemical messages getting around the body.
They cause the heart to slow down and airways to become constricted, leading to suffocation or brain damage.
"It must be excruciatingly painful and unbelievably violent," Dr Sella said.
"You have very painful muscle contractions, vision goes pretty quickly and what little you can see is blurred, then you can't breathe."
What are the symptoms?
Nerve agents, including Novichok, can be inhaled as a fine powder, absorbed through the skin or ingested.
Symptoms can start within seconds or minutes of being exposed and include convulsions, paralysis, respiratory failure and death.
How can it be treated?
The treatment for nerve agents is to administer an antidote immediately, but some of the damage from the chemical and oxygen starvation can be irreparable.
It is not known if there is an antidote available for Novichok.