Paul Glenfield, head of employment law at Matheson Ormsby Prentice, said that having a policy banning the use of company computers for personal use and prohibiting the distribution of inappropriate content is not enough. "Employers should try and think of preventive measures rather than relying on a policy, " he said.

Glenfield said employers should have a strong firewall to prevent unauthorised email reaching employees, and should consider installing software that prevents staff from downloading, or viewing, pornographic images or other inappropriate content on their computers.

The Dublin-based Irish subsidiary of investment bank Merrill Lynch dismissed 13 employees on Friday after they were caught circulating emails containing pornographic material. The bank issued written warnings to another seven people.

The dismissals were the result of a disciplinary investigation that culminated in 20 separate disciplinary hearings at the bank's offices in Dublin last week.

"Our employees are notified of, and advised to carefully follow strict policies on electronic communications", Merrill Lynch said in a statement.

"We want to create the right environment for our employees and do not condone inappropriate use of the firm's systems".

Merrill Lynch spokesman Tim Cobb said the company monitors all its email traffic as a matter of course. The dismissed employees have the right to appeal, he said.

Maura Connolly, head of the employment law group at Dublin law firm Eugene F Collins, said that, once an employer had a clear policy on employee conduct and internet use, and had communicated that to employees, it would be well within its rights to dismiss them for an incident of this nature.

"I think it's fair to say that most employers would not tolerate pornography, " she said.

Connolly noted that in circumstances such as those in the Merrill Lynch case, an employer could face claims from any employees exposed to the pornographic content.

Under both health and safety and equality legislation, an employer has a duty to provide employees with a safe working environment.

"That could, and would, constitute sexual harassment if female employees were to see it, even inadvertently, " said MOP's Paul Glenfield.