Ciaran Casey, Smart's chief operating officer, said the sales rep was dismissed two weeks ago when it was discovered that he was "slamming" customers from other operators.

"We held our hands up. . .we're not making any excuses, " he said.

Slamming is an illegal practice of switching customers from one telecoms company to another without their knowledge. The practice was widespread in the US and other deregulated telecoms markets in the past and has crept into Ireland in recent years as competition between Eircom and alternative operators intensifies.

Last year Euphony, a small telecoms provider, was investigated by communications regulator Comreg after it transferred some 400 customers from competitors without their consent.

The Smart slamming incident involved a small number of customers but has caused some embarrassment for the company as it bids to establish itself as the number one alternative to Eircom.

Casey said Smart had quality controls in place to ensure its sales reps were not mis-selling its services and that the company "regretted" what happened. "Any kind of slamming like that we take very seriously. We can't tolerate that behaviour, " he said.

The company has instituted new practices to ensure it doesn't happen again, Casey said. "We're going to send a signed copy of the customer authorisation form in future and we have upped the percentage of quality calls we make."

A spokeswoman for Comreg said it had discussed the slamming incident with Smart after receiving complaints.

"With any of the operators, if any issue does arise we will speak to them. There have been discussions with Smart and they have rectified the situation, " she said.

Comreg advises that customers who believe they have been switched without giving their permission should ask their provider for details of the authorisation given and contact the regulator.

Meanwhile, the regulator is preparing to square off against Smart in the High Court this Thursday. Smart is challenging Comreg's withdrawal of a third-generation mobile licence awarded to it last year. Comreg withdrew the licence in February, claiming Smart failed to lodge a 100m performance bond.