A military research facility is thought to be conducting tests after Russian double agent Sergei Skripal and his daughter Yulia were exposed to an unknown substance.
The pair are fighting for their lives after being found unconscious in Salisbury, Wiltshire, on Sunday.
The Defence Science and Technology Laboratory in Porton Down, which has state-of-the-art equipment to look for trace amounts of substances, is believed to be involved in examining what could have caused them to fall ill.
Salisbury MP and city minister John Glen told the BBC: "Fortunately just down the road in my constituency at Porton Down defence, science and technology labs exists and they will have taken the substance and will be trying to evaluate what they can, no doubt."
An unnamed ex-radiation biologist said in a comment issued through the Science Media Centre: "If the outcome is not just something like a couple of dodgy kebabs - again less probable with both individuals being symptomatic at the same time and with considerable rapidity if they had just eaten there - then I would look for a chemical source.
"Decontamination at the scene would also suggest that possibility. However, we shouldn't totally ignore biological contamination of food or the environment.
"However, the latter would have caused a wider response from PHE and the authorities."
They added that toxicologists would be looking at samples of both fluids and tissues as well as being told the symptoms.
Alastair Hay, Professor Emeritus of environmental toxicology at the University of Leeds, said it could take some time before test results are available.
He said: "Individuals cannot provide unlimited amounts of blood for testing so investigations will be guided by the clinical team.
"Some tests are rapid and some candidates will be looked at quickly.
"But if the cause is more unusual, body fluids will require significant clean-up preparation before they can be put in an instrument. So this could take a day or several days."