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Islamic State fanatics returning to Britain could be plotting horrifying attacks using chlorine gas which killed thousands of troops during the First World War.

The warning has been given by one of the world’s top experts in chemical warfare, Colonel Hamish de Bretton-Gordon, the retired head of chemical and biological weapons for the British Army and NATO.

After returning from advising security forces in Baghdad last week, he said he feared an attack by returning jihadis using knowledge of chlorine gas-filled IEDs is “highly likely”.

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 90,000 tonnes of chlorine without any licence.

"In a confined space if you saw this yellow and green gas cloud and started smelling it the panic would create carnage.”

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The chilling alert comes days after the 20th anniversary of the attack on the Tokyo subway when extremists released the deadly gas sarin on several tube lines.

And it follows increasing fears IS thugs are stockpiling chemical weapons for a bloody last stand against troops fighting to kick them out of Iraq.

Spies believe the network’s ruthless leader Abu Bakr al-Baghdadi has ordered his followers to encourage an all-out attack in Europe.

(Image: Rex)

Mr de Bretton-Gordon revealed hundreds of crude chlorine IEDs – which cause choking – are now being used extensively by IS in the defence of Tikrit.

Iraqi security forces are convinced chlorine bombs are being prepared to defend Mosul, Iraq’s second city.

When IS overran Mosul last June it seized a huge chlorine factory and Iraqi forces believe it is stockpiling material for a deadly last stand. There are also fears IS has even deadlier mustard gas and sarin.

It would be the first time IS have used chemical weapons on a truly huge scale.

Mr de Bretton-Gordon said: “Islamic State are all over these chemical weapons. The big prize is Mosul, its capital in Iraq. Lose it and they lose the country and have to retreat to Syria.”

The Mirror recently revealed SAS forces in Iraq have been ordered to take chemical weapons suits.

Now managing director of Avon Protection, Mr de Bretton-Gordon has advised the UK government, Iraqi government and Free Syrian Army.

He added: “Syrian President al Assad used sarin in 2013 when about to be over-run. IS have seen this and Mosul could be their last stand. They have no boundaries in awfulness.”

One intelligence source said: “Islamic State definitely wants a spectacular in the West – to build on fear it generated in the Charlie Hebdo attack in Paris.

"Its favourite place would be a transport hub, with footage of the aftermath likely to be broadcast on the internet spreading horror worldwide.”

Next month is the 100th anniversary of the first use of gas in modern war. The German Army deployed chlorine at Ypres, killing 6,000 Allied troops, mainly from asphyxiation and lung damage.