ISIS plotting Sarin nerve gas attack on London's Tube network

The poison killed 13 people and left more than 6,000 sick, injured or temporarily blinded amid the 1995 Tokyo subway attack.

Schiena grabbed headlines last year after trekking into a mountain Kurd community to expose ISIS' use of mustard gas in Iraq.

"A potential chemical attack can’t be dismissed," said Schiena, speaking to Daily Star Online.

"I don't think mustard gas is a likely weapon to be used in the UK.

"But if ISIS was to use a chemical weapon, which is not easy to smuggle or manufacture in-country, it would be a chemical weapon more advanced with increased potential to cause mass casualties.

"A chemical weapon like Sarin released in a subway would not be out of the question and would be crippling to London's transport infrastructure."

Sarin, a colourless and odourless liquid first seen in WWI warfare, is widely considered a weapon of mass destruction and was globally outlawed in April 1997.

Death – due to suffocation of the lungs – can occur within one to ten minutes after direct inhalation.

Japanese religious sect Aum Shinrikyo killed eight people and harmed over 200 in Nagano in 1994.

It then killed 13 on the Tokyo Metro – the last time a terrorist faction released Sarin.

Sarin is already rumoured to be floating around the hands of militants in Iraq and Syria.

The evil weapon is estimated to have killed between 322 and 1,729 during the Ghouta chemical attack amid the Syrian civil war.

Rebels blamed the Syrian government and the government blamed foreign fighters.

Daily Star Online has already reported how Islamic State is using banned chemical weapons after raiding Saddam Hussein's secret WMD stash.

Turkish MP Eren Erdem also claimed that Daesh warmongers snuck Sarin out of Turkey to unleash on the West.

South African-born Tony Schiena, who began his career in the nation's intelligence service amid the Apartheid, has worked as a private intelligence specialist for militaries across the globe.

The mercenary, who happens to be an undefeated world heavyweight karate champion, now heads up MOSAIC – a company devoted to counter terrorism training and counter human trafficking.

His foresight for ISIS attacks have proved creepily accurate in the past.

Schiena – who says an attempted massacre in London is inevitable – predicted an assault on the West on the morning of the Paris attacks on November 13 because they had just lost their Sinjar stronghold in Iraq.

(dailystar.co.uk)