UK mobile provider Three is to be questioned by regulator Ofcom over a data outage, which left customers without 3G services.
Despite apologising for the outage on Tuesday, Ofcom confirmed in a statement to Sky News that it would be investigating the incident.
"Ofcom is aware that Three customers have been affected by a recent mobile data outage on its network," said a Three spokesperson.
"We understand that data services have now been restored but are continuing to liaise with Three to understand the cause of problem."
Repairing the outage by Wednesday, Three claimed that engineers worked all night to resolve the issue, but failed to clarify what problems were to blame for the blackout.
"Our engineers have worked through the night to resolve some issues related to yesterday's internet access problems that affected some of our customers' internet access," said a Three spokesperson.
"All services, including My3 account services have been restored and we are monitoring closely so we can address any performance issues if they arise."
"Voice and text services remained available throughout. We apologise for any interruption of service and for those affected - thank you for your patience."
The network provider issued a further statement to its nine million customers, advising those still experiencing network problems to reset their handsets.