Privacy campaigners expressed shock last night after it emerged that large amounts of confidential personal information held about British citizens on a giant computer network spanning the European Union could be accessed by more than 500,000 terminals.

The figure was revealed in a Council of the European Union document examining proposals to establish a new agency, based in France, that would manage much of the 27 EU member states' shared data. But the sheer number of access points to the Schengen Information System (SIS) – which holds information regarding immigration status, arrest ­warrants, entries on the police national ­computer and a multitude of personal details – has triggered concerns about the security of the data.

Statewatch, a group that monitors civil liberties in Europe, said it was aware of a case in Belgium where personal information extracted from the system by an official was sold to an organised criminal gang.

"It is well known that the greater the points of access, the greater the number of people who have access and the greater the chance that data will be misplaced, lost or illegally accessed," said Tony Bunyan, director of Statewatch. "The idea that mass databases can be totally secure and that privacy can be guaranteed is a fallacy."

The rapid expansion of the EU has played a significant part in increasing the size of the network. In 2003, there were 125,000 computer terminals across the EU with access to the system, according to official documents. But following EU enlargement, the number of computer terminals with access to the system increased dramatically.

According to the Council of the European Union "Inter-institutional File", "the SIS is built around a central database that is networked, via national systems, to more than half a million terminals located within the security services of the member states". The file goes on to explain that the system "currently contains more than 30m alerts [for wanted persons, stolen vehicles and stolen or lost identity papers and documents]". While the SIS is credited by its supporters with helping to track wanted criminals and illegal immigrants, there are concerns that the personal data it holds could be invaluable for fraudsters.