Government scientists are analysing the substance at the centre of the Russian spy attack, amid fears it was a deadly nerve agent used in previous high profile political assassinations. Experts from the government's chemical defence laboratory at Porton Down, just six miles from where Sergei Skripal was targeted in Salisbury, were understood to be urgently trying to identify the chemical. The former Russian spy and his 33-year-old daughter Yulia, who was with him at the time, were rushed to Salisbury District Hospital, where a major incident was declared, amid fears the mysterious substance could result in further casualties. One theory being explored was that the substance could be the deadly nerve agent, VX, which was used last year in the murder of Kim Jong-nam – the estranged half brother of the North Korean leader Kim Jong-un. The chemical was first developed by the British firm, ICI, in the 1950s, but was put to deadly use by Saddam Hussein in an attack against the Kurds in 1988. Yulia Skripal was on holiday visiting her father when they both collapsed in a Salisbury street Kim Jong-nam was killed last February at Kuala Lumpur International Airport, when two women approached him and smeared cloths soaked with VX nerve agent across his face. The fast-acting toxin began to attack his nervous system and despite being able to alert officials, he was dead within 20 minutes. VX, which is the most deadly of all nerve agents, was first developed in the 1950s and is a tasteless and odorless liquid, which can be fatal for humans on skin contact. It penetrates the skin and disrupts the transmission of nerve impulses, leading to a loss of consciousness, paralysis and eventually fatal respiratory failure. Counter-terror police, who are now leading the investigation into the Salisbury poisonings, will also be examining the 2012 death of Russian whistleblower, Alexander Perepilichnyy, who died in mysterious circumstances. Kim Jong-nam was assassinated with the nerve agent VX Credit: Shizuo Kambayashi /AP He was initially thought to have died of natural causes while out jogging, but traces of a deadly chemical was later found in his stomach. It was later suggested he could have been poisoned using the plant Gelsemium elegans, which had been secreted in the sorrel soup he ate shortly before he died. Hamish de Bretton-Gordon, a former commander of the British Army’s chemical, biological, radiological and nuclear regiment, said the attack had all the hallmarks of a toxic airborne nerve agent. A police tent covers the spot where Sergei Skripal and his daughter collapsed in Salisbury Credit: Frank Augstein /AP One theory being explored by counter-terrorism experts is that the substance was contained in an aerosol that was sprayed in the faces of Mr Skripal and his daughter. Officials have said there is no danger to the wider public, but have warned emergency workers who attended the scene, to be wary if they develop itchy skin or eyes or suffer from breathing problems. Video: CCTV shows 'persons of interest' in Salisbury Mr de Bretton-Gordon, who is now chief executive of Avon Protection Systems, told The Telegraph that VX was one possibility in the Salisbury incident. He said: “I certainly don’t think it’s a radiological isotope like polonium-210 that we saw with, Alexander Litvinenko, mainly because it takes considerable time to take effect. “With Litvinenko it wasn’t obvious for two or three days. It would appear that whatever they took or were given or were attacked with, is rather quicker than that.” Alexander Litvinenko, pictured in a UK hospital bed, was poisoned by a radioactive isotope Credit: PA Witnesses reported seeing Mr Skripal holding his hands in the air and shaking violently after he collapsed. Mr de Bretton-Gordon said: “The shaking hands, and also I have also seen it described as him appearing to be frozen, that is sort of what nerve agents do, because they destroy your nerves.” He also said the precautions taken by police, including the use of protection suits, is what would be expected, if they thought they were dealing with a toxin or nerve agent. The other deadly toxins under the microscope Other deadly toxins which are likely to be tested for include the likes of Anthrax and Sarin, will be being tested for. Anthrax Anthrax, which is an infection caused by a bacteria, is invisible and odourless and can cause a painful death if inhaled into the lungs. In less concentrated doses it can cause severe itchiness and shortness of breath. Emergency workers, including the police who attended Mr Skripal, were warned be alert if they developed either of these conditions. Sarin Sarin, which was originally developed in Germany in the 1930s as a pesticide, is used in chemical warfare as a nerve agent. It can be absorbed through the skin, or by breathing it in and exposure to large amounts can lead to a painful death.