Users of the '3' mobile phone network were left without internet access after technical problems hit the network for three hours yesterday.
Customers across the city were affected from about 11am because the mobile data network software components became destabilished, the firm said.
However, its text messaging and normal phone services were not affected, and the internet service resumed at about 2pm.
The incident is the latest stoppage of mobile services to affect Hong Kong in recent months, and internet experts have urged telecommunications firms to conduct regular checks on their networks. 3 Hong Kong, run by Hutchison Telecommunications, has apologised and said it was investigating the matter and would seek to prevent a recurrence.
A spokeswoman for the Office of the Communications Authority said it was notified about two hours after the incident occurred and that it had received 121 complaints by last night.
The company has been given until Wednesday to submit a preliminary report on the incident.