The mystery substance which struck down Sergei Skripal and his daughter is unlikely to be radioactive and could be a biologicial toxin such as ricin, according to a chemical expert.
The 66-year-old former Russian colonel and his daughter Yulia, 33, are fighting for their lives in hospital as scientists race to determine the substance they have been exposed to.
Reports questioned whether the suspected poison was radioactive, such as the polonium used to kill Russian dissident Alexander Litvinenko, a nerve agent or a toxin.
But Hamish de Bretton-Gordon, a former commander of the Army's chemical, biological, radiological and nuclear regiment, said reports of the incident pointed away from radiation poisoning.
Referring to the comments made by one official to BBC's Newsnight that the pair were being treated for "symptoms rather than causes", Mr De Bretton-Gordon said that was "quite significant".
"It's obviously some sort of poisonous substance," he said.
"I don't think it's polonium - the incubation period is too long and the decontamination that's been reported is not the kind you would do for a radioactive isotope.
"If Porton Down are saying they are treating symptoms rather than causes they are doing palliative care.
"If it was a nerve agent they would be given an antidote, which leads me to believe it is probably not a nerve agent.
"Porton Down have no doubt taken blood samples and they should know by now what it is.
"Toxins like ricin - there's no antidote for that. If people are poisoned with ricin or abrin you treat the symptoms."
Ricin has been used in assassinations before, notably in September 1978, when Bulgarian dissident Georgi Markov was killed in London using a poison-loaded umbrella.
The murder involved a minuscule dose of ricin, which is produced naturally by the castor bean plant.
But Mr de Bretton-Gordon said weaponising ricin was a complex process that would "probably need a professional laboratory to do it properly".
"Ricin can kill you very quickly - the longer they survive the better chance they have got.
"Poison does not get worse, you either die or you survive."
Ricin was easier to get hold of and could be planted in food or drink, Mr de Bretton-Gordon said.
He added the use of nerve agents such as Sarin or VX, which was used to kill Kim Jong Un's half-brother Kim Jong Nam, would likely have killed the pair quickly, while reports of thallium, a highly toxic odourless and tasteless heavy metal, were also "possible" but would require a relatively large dose which was an "imprecise science".
And he said: "If you were going to be poisoned anywhere in the world you would want to be near Porton Down."
Mr de Bretton-Gordon said the facility, around eight miles from Salisbury, was the "leading toxicology lab in the world" which hold the supplies for the military.
"They have all the counter measures," he said.