Returning Islamic State militants could launch a deadly gas attack on London's public transportation system, a sporting event, or where large crowds are gathered, a chemical weapons expert claims.

"It is very evident that [ISIS is] putting much time and effort into training its jihadis in the use of chlorine as a terror weapon and in particular in IEDs [improvised explosive devices]," wrote Hamish de Bretton-Gordon, on the website 2Paragraphs.

The past two weeks has marked the "most concentrated and deadly use of chemical weapons" since the 1980s Iran-Iraq War, said de Bretton-Gordon, and the jihadist training will make the militants returning home much more dangerous.

"Virtually every foreign jihadi who returns to the United States or United Kingdom will have been exposed to training of this sort and will have a reasonable idea on how to use chlorine and other toxic chemicals as a terror weapon," he wrote. "In the UK, up to 90 tons of chlorine can be purchased without any licenses."

The expert told The Daily Mirror after returning from his role advising security forces in Baghdad last week that such an attack is "highly likely," and could happen "on a train or tube or even at a big football match."

De Bretton-Gordon, is a veteran of the British Army, where he served as commanding officer of the UK's CBRN [chemical, biological, radiological, and nuclear] Regiment and NATO's Rapid Reaction Battalion.

He has recently worked with the charity Syria Relief, advising on what to do during a chlorine gas attack, and founded the CBRN defense firm SecureBio. He is also a director at gas mask manufacturer Avon Protection.

He compared the potential for an attack in London to the sarin attack on the Tokyo subway 20 years ago that killed 12 and injured more than 1,000. For their attack, reports The Independent, terrorists with the Aum Shinrikyo movement punctured packets of the sarin nerve agent with umbrella tips while on the Tokyo subway during the morning rush hour.

But a chlorine attack would be much simpler to pull off, de Bretton-Gordon wrote for 2Paragraphs: All a terrorist would need to do is "take the top off a chlorine canister and let it 'vaporize,' aka 'weaponize.'"

ISIS got access to chlorine when it gained control of a large factory near Mosul, and also when it took over the complex near Baghdad where Saddam Hussein had been making chemical weapons, including sarin, mustard gas, and VX, said de Bretton-Gordon.

The militants have planted "hundreds" of chlorine IEDs to defend Tikrit, he said, and also detonated chemical bombs against the Kurdish Peshmerga.

But de Bretton-Gordon feels the effect of a chlorine attack in Britain will be minimal if security services are warned.

"Chlorine is not very toxic and the green and yellow clouds are easy to see and avoid, and it is very nonpersistent, only lasting for a few minutes," he said.

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