The American-made software, called WebSENSE, was installed to prevent students seeing pornography, or bomb-making instructions. It searches sites for prohibited words such as "sex".
But the TUC site was ruled out of bounds after the software decided it repreented "activist groups", defined as "organisations that promote change, or attempt to influence opinion..."
The prudish program also prohibits access to sites about a Japanese baseball team, The Swallows, and a grocer who boasts he sells fresh as well as frozen food.
But at least one union managed to escape suspicion of trying to change opinion. When Stoke lecturers tried to visit the NATFHE site, they succeeded.