Terror chiefs believe a devastating chemical weapons attack in Britain is now ‘more likely than not’, The Mail on Sunday can reveal.

The chilling assessment follows the interception of ‘chatter’ between senior figures in Islamic State (IS). The terror group has been inspired by the poisoning of former KGB agent Sergei Skripal and his daughter, Yulia, by Russian agents in March.

Before the novichok attack in Salisbury, the Government’s Joint Terrorism Analysis Centre (JTAC) put the risk of a chemical weapons strike by jihadis at 25 per cent.

Security sources say that has now surged to more than 50 per cent. There are particular fears over the potential for a chlorine bomb to be detonated on the London Underground. The threat is considered so severe that terror chiefs secretly met with emergency services bosses a fortnight ago to ‘war game’ their response to such an atrocity.

Terror chiefs believe a devastating chemical weapons attack in Britain is now ‘more likely than not’ (pictured: Emergency services in Salisbury, March 8)

Teams, including officers from the Met Police’s Emergency Preparedness Operational Command Unit (CO3) and officials from the London Mayor’s office, were faced with a scenario of simultaneous attacks at Oxford Street and Waterloo Underground stations by terrorists carrying chlorine bombs hidden in rucksacks.

When such devices are triggered, the relatively harmless liquid chlorine becomes a deadly vapour that mixes with fluid in the lungs and eyes of victims to form corrosive hydrochloric acid. The gas would be particularly dangerous in confined and densely packed Underground stations, especially for children and the elderly.

The recent simulation involved commuters being killed as the chlorine gas swept through trains and along platforms. Many more ‘died’ as terrified passengers fought to escape. It was concluded that up to 100 lives could be lost in such an attack, with hundreds more injured.

Islamic State has been inspired by the poisoning of former KGB agent Sergei Skripal and his daughter, Yulia, by Russian agents in March

Before the novichok attack in Salisbury, the Government’s Joint Terrorism Analysis Centre (JTAC) put the risk of a chemical weapons strike by jihadis at 25 per cent (stock image, King's Cross St Pancras)

A security source involved in the exercise said: ‘The chlorine vapour would be very localised and would last a few minutes before it evaporated. While fatal, the stampede to get out of the Tube station would cost far more lives than the chemical. That’s why it is important to educate people about the threat of these weapons. The more they know, the less inclined they’ll be to panic.’

Last night, Hamish de Bretton-Gordon, an expert on chemical weapons, said IS bomb-makers in Syria had already developed the necessary skills to make such devices and could pass such expertise to extremists in Britain.

He said: ‘These tactics have been morbidly successful for IS in the Syria war zone, while the nerve-agent attack in Salisbury has shown that just a tiny amount of a chemical can have a huge impact.’

Responding to The Mail on Sunday’s exclusive report, Security Minister Ben Wallace said last night: ‘I have consistently warned that a chemical attack in the UK is getting more likely. We have well-tested plans to respond to an attack and minimise the impact, should an incident occur.’