GCHQ's move was complicated by a huge computer network

Engineers who designed the computer system for the new headquarters of the spybase in Cheltenham failed to realise how complex their computer systems had become.

Giving evidence to the Commons Public Accounts Committee, GCHQ director David Pepper explained when the plans were drawn up in the mid-1990s it was not understood how interdependent the computer systems had become.

Earlier this year, a National Audit Office report revealed a failure to appreciate the difficulties involved meant the cost of the move to the so-called 'doughnut' in Cheltenham rocketed from an original estimate of �41m to �308m.

Networking systems

Dr Pepper said the original plans to transfer the computers were based on systems used during the Cold War.

Then, there was a relatively static threat from the Soviet Union and the different computer systems used by GCHQ staff did not have to interact.

But more complex threats facing the UK in the 1990s meant more "networking" between computer systems, making the move much more complicated.

"The engineers simply didn't spot the fact that the degree of networking was going to complicate enormously the process of transition," he told the committee.

"It took some time before anybody stood back and said 'It is not going to work because of the networking'."

He said that it was only in 1998, during work to protect GCHQ's computers from the "Millennium bug", that the extent of the problem was realised.