Terror group ISIS is training hundreds of British jihadists to carry out what a chemical expert called a "highly likely" chlorine gas attack on trains and sports games in the U.K., a report said.
"I am convinced that IS (another name for ISIS) fighters are all being given training in chemical weapons and the British ones, who are likely to be more educated, will all be targeted in the hope they may return home," said Colonel Hamish de Bretton-Gordon, the retired head of chemical and biological weapons for the British Army, according to the Mail Online.
"They will have a reasonable idea on how to use chlorine and other toxic chemicals as a terror weapon," he added.
"This could happen on a train or tube or even at a big football match. Acquiring weapons and ammunition is very difficult in the UK but you can get up to 90 tonnes of chlorine without any license."
Scotland Yard has apparently warned that close to 500 Britons trained by ISIS in Iraq and Syria have returned to the U.K., raising fears of a terror attack at home.
Chlorine is a "choking agent," which can burn the lungs when inhaled in large quantities.
Colonel de Bretton-Gordon, who is now managing director of Avon Protection, added that he has been teaching Syrian civilians on how to avoid chlorine, and is sharing the same advice with Britons.
"Chlorine is not very toxic and the green and yellow clouds are easy to see and avoid. It is very non-persistent only lasting for a few minutes. It was used in the First World War but dropped because it did not work very well," he continued.
"It is key that ISIS don't get the advantage of an unexpected chemical attack in the UK. If you can hold your breath for 30 seconds and run in the opposite direction you will be okay. If outside you should aim to climb or reach higher ground."
Britain has noted that a large number of its citizens are going oversees to fight for ISIS. Back in August 2014, Foreign Secretary Philip Hammond revealed that an estimated 800 British Muslim fighters have joined the terror group, which was more than the 600 or so British Muslims serving in Britain's Ministry of Defense Armed Forces.
ISIS has attracted over 20,000 foreign fighters to fight for its mission of establishing an Islamic Caliphate in the region.
One infamous British terrorist has been identified as "Jihadi John," a man seen in a number of ISIS' beheading videos. British intelligence believe the man is Mohammed Emwazi, a Kuwaiti-born computer-programming graduate from London.
British Prime Minister David has vowed to do "everything we can" to bring to justice terrorists such as "Jihadi John."
"When there are people anywhere in the world who commit appalling and heinous crimes against British citizens, we will do everything we can with the police, with the security services, with all that we have at our disposal to find these people and put them out of action," Cameron said back in February.