Eircom and ComReg are arguing over the telecoms firm's decision to move some mobile customers to online billing.

ComReg says Eircom's mobile arm breached its obligations to notify customers of a change to contracts when it introduced the new billing method.

It has given Eircom three weeks to answer ComReg's breach notice, which was sent to the company on Friday and published today, or face enforcement action.

About 207,000 Meteor and eMobile customers were moved to online billing during August.