The Doughnut that is home of Britain's best kept secrets

The Government Communications Headquarters, or GCHQ, has long been seen as the most secretive of Britain's three intelligence services.

It works alongside MI5 and MI6, gathering and analysing intelligence, and its staff of about 5,500 provide technical advice to the Government on protecting its communication and information systems.

The GCHQ building in Cheltenham - known locally as the Doughnut - houses some of the most powerful computers in western Europe. Most of its work is for Ministry of Defence, the Foreign and Commonwealth Office and law enforcement agencies and its director, Iain Lobban, reports to the Secretary of State for Foreign and Commonwealth Affairs.

GCHQ was established after the First World War as the Government Code and Cypher School (GCCS or GC&CS), by which name it was known until 1946. During the Second World War it was based largely at Bletchley Park, where the code to the German Enigma machine was broken.

GCHQ has placed job ads in the virtual world of computer games in a bid to get technically adept youngsters to apply.