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.
In early April, users of the SmarTone network were unable to use their handsets to call or use the internet due to a power failure at a switchboard station in the Central and Western district. The stoppage affected about 243,000 customers.
In a report to the regulator, SmarTone said the cause of the generator breakdown could not be identified 'even after a comprehensive inspection'.
But with some 15 million mobile phone subscriptions active in the city - representing 2.1 phones each for every man, woman and child - experts say the telcos must do better.
'The telecommunications companies should carry out the checks on their systems carefully and regularly. They should also pay attention to the investigation reports issued by the authority,' said Charles Mok of the Internet Society Hong Kong.
SmarTone's number of mobile users in Hong Kong