Staff have begun moving in to the government's new spy base at Cheltenham.
The doughnut is bigger that the old Wembley Stadium
Nicknamed "the doughnut" because of its unusual shape, GCHQ's new headquarters took three years to build at a cost of £337m.
In July, the agency was criticised by the Audit Commission for massive cost overruns in the move.
All 4,500 staff began moving in on Tuesday.
Bob McNally, spokesman at GCHQ, told the BBC: "Previously, we had about 50 buildings separated over two sites four miles apart.
"It was very difficult for people to get together and work.
"The new building is open plan with common computer systems. People can get together in 15 minutes."
The doughnut was built on a circular design, big enough to house the old Wembley Stadium, including a courtyard large enough to take the Royal Albert Hall.