The programs work by monitoring all employees' web and email activities via the company's main server. Once it detects an unusually high number of flesh-coloured pixels in a downloaded image, it alerts the employer that illicit material is being downloaded.

Other programs can identify key words and phrases in emails which will tell the employers whether the employee is downloading and disseminating racial or sexually abusive material in the workplace. An employer risks prosecution if he allows illicit material, which others would find offensive, to be dispersed via company computers.

But Duncan Inverarity, a partner in employment law firm BCM Hanby Wallace, warned that while employers have a right to protect themselves against litigation, if they are going to monitor their workers' internet activity they should let employees know that this is being done.

Employers are entitled to monitor web activity and intercept emails, but it has to be done in accordance with strict privacy rules laid down by the Data Protection Commissioner.

Inverarity instanced one recent case where an IT manager intercepted the managing director's emails to find out what other senior managers were being paid. He was dismissed, but his claim for unfair dismissal was settled because it was not made clear that the IT manager could not access another employee's emails.