The company regularly monitors workers' internet use

The firm, which monitors the websites its staff log on to all year round, said it made the discoveries during a recent crackdown on internet abuse.

Rolls Royce said the material being accessed was not illegal and the police have not been called in.

But Hannah Reid from the TUC said the case highlighted serious concerns.

She said: "The TUC is very concerned that increasingly, employers are carrying out monitoring in the workplace, and they've done that without informing members of staff.

"From the Information Commissioner's Office, there are clear laws which actually restrict the extent to which employers can carry out monitoring in the workplace."

A spokesperson for Rolls Royce said the issue has been dealt with in a firm but fair way.

Three workers at the company's East Kilbride plant in Scotland were also sacked for internet misuse.