(Mirror) — GOVERNORS at Britain’s top terror jail believe al-Qaeda extremists are planning a major attack as payback for Osama bin Laden’s death.

Security has been stepped up to “unprecedented” levels at Belmarsh prison after warnings that the Muslim Boys gang is plotting a revenge mission. A 15-strong riot squad in stab-proof vests has hidden around the jail’s chapel during Muslim prayers on Fridays in case of an attack.

Another team of officers has been on standby ready to assist.

Prisoners on House Block One, which includes 39 terror suspects, added to the tension by staging a mass show of defiance on Friday.

Inmates refused to return to their cells and a source says “staff very nearly lost control of the jail”.

The standoff was only ended when officers brandished batons and demanded a retreat as they marched towards the prisoners.

The source said: “It’s a powder keg waiting to go off.

“There are concerns it will take an officer’s death before the Muslim Boys are tackled seriously.

“The levels of security being employed are unprecedented.Management are worried the Muslim Boys are planning an incident in Bin Laden’s honour on a scale never seen before.