Ofcom has issued Pipex with a warning after it was found that the provider had failed to provide customers with the MAC code needed when switching broadband.
The code is required to avoid interruption to a service when subscribers decide to switch providers.
Ofcom brought forward a regulation in 2007 which required customers to be provided with their code within five working days of receiving a request.
According to the communications watchdog: "Today's (Wednesday, December 17th) notification highlights a number of occasions where Pipex failed to abide by these rules and requires them to ensure that they are compliant by the deadline of January 23rd, 2009.
"Ofcom will closely monitor Pipex's performance to ensure that the company does now comply with the rules."
Ofcom has said it will consider "enforcement action" if the deadline is not complied with.