DBS' digital banking services resumed yesterday after a disruption that prevented customers from accessing services through its website and app for about two days.
The outage was not caused by a cyber attack, said the bank.
In an update posted on Facebook at 9pm yesterday, DBS said customer log-ins and transaction activities had returned to normal pre-disruption levels since yesterday morning, though some customers were still facing issues.
"Although our digital banking services have returned to normal, the inability to access an essential service over such an extended period of time is unacceptable... Once again, we recognise the gravity of the disruption. We apologise for the inconvenience and anxiety caused," the bank said.
It urged customers who could not log in or perform selected transactions to restart their devices or clear their browser cache before logging in again.
Several companies The Straits Times spoke to said there was minimal disruption to their services.
A Great Eastern (GE) spokesman said: "We have not seen any impact on existing Giro arrangements, payment processing or insurance payouts to policyholders via direct crediting or PayNow to their linked bank accounts."
The spokesman added that GE will advise customers about alternative payment modes and extend grace periods for premium payments where necessary.
"So far, we have only received an enquiry from one customer on this, and we are assisting accordingly," the spokesman added.
Ms Lee Tsui Lin, Prudential Singapore's head of operations, said the insurer provides customers with multiple options for premium payments.
"We also have a grace period for premium payments to ensure that customers have sufficient time to complete their payments in such scenarios," she added.
Mr Roy Kee, managing director of cleaning products manufacturer JRW International, said the disruption was not keenly felt as the company still uses cheques for some payments, and also banks with other lenders besides DBS.
"As a new convert to e-banking, this will shake my confidence a bit. The bank could have also communicated with clients better by sending us SMS messages about the disruption," he added.
The disruption to the website and mobile app services started on Tuesday morning.
Although customers said they could log in to its digibank online platforms yesterday morning, many still could not make transactions or view past ones.
After restarting her phone, Ms Allison Teo said she could log in to the DBS digibank app around noon yesterday to transfer money to a friend via PayNow.
"It was just a small amount as she had helped to purchase a gift for a friend who was discharged from hospital. It wasn't really urgent, but I don't like to delay payment," said Ms Teo, 42, a programme executive.
Reacting to the bank's worst outage in more than a decade, the Monetary Authority of Singapore (MAS) said on Wednesday evening that it would consider taking "supervisory action".
Under the central bank's regulations, financial institutions must ensure that the total unscheduled downtime for critical systems affecting services for customers does not exceed four hours within any 12-month period.
Banks must also inform MAS of any incidents affecting critical systems within an hour.
In a Facebook video post on Wednesday, DBS country manager for Singapore Shee Tse Koon apologised to affected customers for the inconvenience, and blamed faulty access control servers for the outage.
He did not elaborate on the server fault.
Access control servers are part of a bank's security system. They handle both log-in and payment verification using means such as biometrics, authentication tokens and one-time passwords.
The disruption is the worst DBS has seen since 2010, when a major systems failure took down all consumer and business banking services at the bank.
Customers were unable to withdraw cash from ATMs or make point-of-sale payments for about seven hours.
DBS shares slid 0.83 per cent to $32.11 yesterday.