Orange, the British mobile phone operator, yesterday sacked 45 employees in one of the biggest crackdowns yet on staff Internet use.

The workers - at call centres in Darlington and Peterborough and offices in Hertfordshire - were dismissed for distributing to colleagues pornography downloaded from the Internet.

The company said the offenders had broken its rules against the creation and exchange of messages that were "offensive, harassing, obscene, racist, sexist, threatening or libellous". It said the dismissals followed a thorough investigation of its internal computer systems, but it would not say if staff were warned before their dismissal.

According to employment lawyers and corporate investigation companies, cases of employees being sacked for alleged misuse of the Internet are increasing as organisations tighten up their internal rules.

Employment law specialist Mr Tim Russell said it was important for companies to act consistently and give clear guidelines to employees to avoid any disciplinary action backfiring in litigation.

He added, however, that defining the misuse of computers remained complex. For example, employers could be held liable for claims of female sexual harassment if an in-house system was used to download and distribute pornography.

"Without clear guidelines, however, companies could face a human rights issue of privacy," Mr Russell said.