Prudential said a technical glitch had caused the Giro banking glitch.

By 11am it had received about 300 calls from worried customers, including Madam Ann Chow, who realised that her bank account was short of $320 when she withdrew cash at an the ATM on Wednesday morning.

'I immediately checked my account's past transactions via Internet banking. The premiums of two policies were deducted twice on Tues. I was alarmed and called the hotline,' she said.

Madam Chow pays monthly premiums of $160 for each of her two Prudential policies - an investment-linked and an endowment plan. Instead of the usual monthly premium deduction of $320, $640 was deducted this month.

An officer at the customer hotline assured her that the money will be reinstated.

'I was so glad that it wasn't just me and I will get my money back. I even forgot to ask when it will be reinstated,' she said.

Prudential told The Straits Times: 'We sincerely apologise for the error and the inconvenience caused to our policyholders. We will reverse the extra deduction back to their bank accounts by Dec 26.'

One of the top three life insurers, Prudential has been here for 77 years, and it has about 500,000 customers. Besides its 3,400-strong agency force, Prudential has exclusive partnerships with Standard Chartered Bank, Maybank and Singapore Post.

Prudential declined to give further details on the glitch. It is still investigating the source of the error and the number of affected customers.

It is believed that only transactions done on Tuesday were affected by the glitch.

Despite technological advances, glitches do happen.

Last week, computer problems at the Singapore Exchange caused a lag in price updates and order processing which affected all dealing houses.

The problem started in the late morning and dragged on for most of the trading day.