Over a thousand branches of Natwest and the Royal Bank of Scotland will open early today to help customers who are still experiencing problems after the massive computer outage on Cyber Monday.

An estimated 750,000 customers said they could not use cash machines, debit cards or access online or telephone banking on Monday evening.

The RBS group, which also includes Natwest and Ulster Bank, has said that its 15.7 million customers should contact its helplines or visit their local branch if they were still experiencing problems caused by the subsequent backlog from Monday, which was predicted to be one of the busiest online shopping days of the year.

The bank has promised that anyone who experienced problems during the computer outage will be compensated for their losses accordingly.

RBS also said it will get to the bottom of recurring IT problems.

RBS's chief executive Ross McEwan has blamed decades of IT system investment failure for the customer banking black-out.

"For decades RBS failed to invest properly in its systems. We need to put our customers' needs at the centre of all we do. It will take time but we are investing heavily in building IT systems our customers can rely on.

"I'm sorry for the inconvenience we caused our customers. We know we have to do better. I will be outlining plans in the new year for making RBS the bank that our customers and the UK need it to be," he said.

In August 2012, RBS revealed that it had put aside £125m (€150m, $204m) to compensate thousands of customers affected by a two-week computer breakdown, which led to millions of customers being stranded and unable to pay outgoing bills.

In April this year, Britain's Financial Conduct Authority said it will investigate RBS over its 2012 computer failure that left millions of customers unable to access their accounts.

RBS was not immediately available for comment.