MPs have condemned the spiralling cost of moving the Government Communications Headquarters listening post (GCHQ).
GCHQ's move was complicated by a huge computer network
The spybase moved to new headquarters, the "doughnut" in Cheltenham, last year, after occupying more than 50 buildings on two sites since the 1950s.
The transfer was completed under a £489m private finance initiative.
But GCHQ moved its technical capacity itself for security reasons, and the estimated cost of the move rose from £41m to £450m in just two years.
The Treasury refused to fund the rise but paid £216m towards a revised budget of £308m.
On Tuesday, Commons Public Accounts Committee chairman Tory Edward Leigh, said: "It is astonishing GCHQ did not realise the extent of what would be involved
"The process of moving the technology so critical to GCHQ's business will take longer and cost well
over £300m more than initially estimated."
The committee said it had been a mistake to assume a private finance initiative had been the only way to proceed.
And GCHQ had made a "highly uncertain assumption" that a
conventionally procured building would have over-run its budget by 24%.
It had also been "unwise" to enter negotiations with a preferred
bidder when key requirements had not been settled or priced.
During almost two years of exclusive negotiations, the price rose 21% before the contract was even signed.