MI5 fear that militant Islamists will attempt to exact revenge on the British spy, who penetrated al-Qaeda in the Arabian Peninsular (AQAP), by publishing his photograph on the internet – a move designed to incite extremists to hunt him down.
Sources have described the British spy as "gold dust", adding that he was one of just a handful of agents in the last ten years to have successfully penetrated one of the groups aligned to al-Qaeda's concept of global Jihad.
AQAP now represents the "greatest operational threat" to Britain and America, according to senior Whitehall sources.
The group is known for its use of modern communication techniques including the publication of an English-language magazine, Inspire, which is distributed to supporters over the internet.
The agent, a British passport holder of Saudi heritage, volunteered to take part in a suicide mission but instead escaped with an underwear bomb designed to blow up a US airliner.
He is understood to have been recruited and trained by MI5's G6 section – the ultra-secret part of the organisation responsible for agent handling – before being sent on his mission to penetrate the Yemeni-based terror group.
A former security official told The Sunday Telegraph that although the mission to penetrate AQAP was a success, the agent was now "burned" and would never be able to take part in covert operations again.
In all likelihood, the official said, the agent will have to be relocated outside of the Middle East and provided with a new identity.