Many M1 mobile users were cut off from the Internet for several hours on Monday morning following a scheduled network upgrade the night before.
In a statement on Monday evening, an M1 spokesman said that this led to "intermittent 3G data access issues".
"Our preliminary findings point to a software bug that prevented some customers from connecting to our mobile data network, and our engineers successfully resolved the issue by 2:00pm," the spokesman said. Customer access to voice calls, text messaging and 4G data services were not affected, he added.
M1 said the problem started at about 8am, but some affected customers reported the problem started as early as 1am. Affected locations included Choa Chu Kang and Bukit Timah to Punggol and Toa Payoh.
M1 first acknowledged the "intermittent" mobile data access problem at about 9.30am on Facebook with an apology. It updated customers with another post at 1pm saying its engineers were "working hard to resolve the issue", without providing details.
At about 3pm, M1 said on Facebook that it had rectified the problem, but no details were given.