An MI6 double agent who once foiled a planned chemical attack on the New York Subway says he believes the war on terror is far from over.

Aimen Dean was a top spy for the UK intelligence agency during the war on terror.

While undercover, he became one of Al Qaeda's most respected bomb-makers, swore allegiance to Osama bin Laden and rubbed shoulders with the mastermind behind the 9/11 attacks.

He worked on the regime's chemical weapons programme and foiled attacks on civilians, saving countless lives while risking his own.

In an interview with Channel 4 News promoting his new memoir, Nine Lives: My time as MI6's top spy inside Al-Qaeda , Mr Dean said he was not worried about revenge attacks against him following the poisoning of former Russian double agent Sergei Skripal in March.

Aimen Dean says he foiled many terror plots during his time as an MI6 double agent (
Image:
Channel 4)
The double agent swore allegiance to Osama bin Laden during his time in Al-Qaeda (
Image:
AFP)

"The difference (between Skripal and I) is that he is pursued by one of the great powers of the world, and many of my former comrades are either dead or in prison.

"I take precautions. I’m not overly worried, but I take precautions to make sure that I don’t end up somewhere where there is a heavy concentration of my ex-associates."

While working as a double-agent, Mr Dean said he reported back to the UK on locations of Al-Qaeda camps, the regime's new training methods and associates and its weapons of mass destruction program.

Despite meeting one of the men who would go on to become one of the 9/11 hijackers, Mr Dean said the terrorist attack could not have been prevented with the intelligence MI6 had at the time.

"If you wanted to stop 9/11, you have to have at least 12 spies inside Al-Qaeda and each one inside one of the different departments. Only then you could have stopped it.

Mr Dean has released his memoir just months after Russian former double-agent Sergei Skripal and his daughter Yulia Skripal were poisoned

"I don’t know (how many spies MI6 had) to be honest, but I will be surprised if there are more than two or three."

Speaking of his coups during his work as a double agent, Mr Dean said he helped prevent a chemical attack on the New York Subway.

"There was a plot to attack the New York Subway with chemical weapons, a chemical device it was invented during my time actually, in that specialist lab in Afghanistan.

"And I was reporting on it regularly to the UK secret services."

Although his work helped prevent some terrorist attacks, the former spy believed the war on terror would continue for years.

Mr Dean says he believes the war on terror is far from over (
Image:
Channel 4)

He said: "I think this will go on for years to come. First of all, the waves of migration coming from the Middle East and North Africa bringing with them basically untold number of risks, and potential threats to this country and Europe in general, this will be a threat for many years to come.

"Especially with radicals being released from prison, with more resentment towards this country, and possibly even greater intent."

Asked of how he felt about betraying his Al-Qaeda associates, Mr Dean said it was not betrayal in the eyes of God.

"Imam Ali said that loyalty to the treacherous is treachery in the eyes of God.

"Betrayal of the treacherous is true loyalty in the eyes of God.

"No. I did not betray a state, I betrayed a bunch of criminals, it’s as simple as that."

According to his memoir blurb, Mr Dean became a double agent after the young Muslim man, determined to defend his faith, found himself fighting on the wrong side.