M1 customers found they could not surf the Web, make calls or send text messages from as early as 3am on Tuesday morning.
The telco's 3G network was down due to a power problem that set off its network centre's gas suppression and water sprinkler systems, which in turn affected one of its mobile network switches, M1 said in a statement Tuesday evening.
"This affected the 3G network entities connected to the switch and resulted in the loss of 3G mobile services to some of our customers in the South-Western parts of Singapore, namely West Coast, Jurong and Tuas areas," said the statement.
Affected customers' phone should automatically switch to its 2G network, M1 said. Customers on its 4G service have to manually turn off the 4G service to switch over to its 2G network. M1 said it was in the final stages of testing the migration of traffic from the affected switch. "Actual migration of traffic from the affected switch will take place progressively," it added.
"We sincerely apologise to our affected customers for the inconvenience caused, and thank them for their support, understanding and patience. Please be assured that our top priority is to resolve this issue as quickly as possible," said its chief executive Karen Kooi.
Meanwhile, angry users flooded the M1 Facebook page with comments, with service outages reported in many areas, including Choa Chu Kang, Ang Mo Kio, Yio Chu Kang, Yishun and Jurong West. Users claimed they could not make calls or send text messages even after switching to 2G with some asking for compensation from M1.