There may be high fences and security cameras around the building site in Berlin, but that wasn't enough to prevent the blueprints for one of the city's biggest construction projects from going missing.

The site is for the headquarters of Germany's answer to M16, making the loss all the more embarrassing.

The spy agency is facing difficult questions after it emerged that it could not even keep the plans for its new hi-tech offices from going astray. According to a report in Focus magazine, the blueprints contained sensitive information relating to the security of the Berlin headquarters.

The government has set up an investigation and requested a revision of security measures at the site. "It's a serious issue and the government is interested in clearing up this case as quickly as possible," said Chancellor Angela Merkel's spokesman Steffen Seibert. The blueprints showed highly sensitive areas of the headquarters, including its logistical nerve centre, anti-terror installations, emergency exits and alarm systems.

The data, most likely stored on a USB stick, was stolen a year ago, public broadcaster ARD reported on Tuesday, citing an unnamed government official. The core of the building may now have to be redesigned, the TV station reported. The security leak is a huge embarrassment to the Bundesnachrichtendienst (BND), Germany's foreign intelligence agency. It is based in Pullach, a Bavarian village near Munich, but the government decided to move the agency to the capital following the terror attacks of 11 September 2001.

The cost of the project had been estimated at €500m (£440m) but had risen to around €1.5bn. The headquarters, which will be located right next to where the wall once stood in the former east Berlin, are expected to be completed by 2014 and will house some 4,000 agency staff.Wolfgang Bosbach, a member of Merkel's conservative Christian Democrats and chairman of the parliamentary domestic affairs committee, expressed deep concern about the vanished blueprints.

"This is a serious incident," he told the Guardian, adding that it is still not clear if the blueprints had been stolen or mislaid due to "sloppiness".

"The big worry is that these plans have fallen into the wrong hands," he said. "We could also lose the trust of our foreign partners, who may think: how can we give the Germans information and documents if they don't look after them carefully?"