The glitch left customers of RBS and its NatWest subsidiary unable to use credit and debit cards for three hours yesterday evening while the banks’ websites and smartphone apps were also affected.
Mr McEwan said: “Last night’s systems failure was unacceptable. Yesterday was a busy shopping day and far too many of our customers were let down, unable to make purchases and withdraw cash.
“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. This will include an outline of where we intend to invest for the future.”
The IT failure left some customers unable to pay after filling up with petrol or to settle restaurant bills while others were forced to abandon full shopping trolleys at the check-out. It was the latest in a string of computer meltdowns for RBS.
It came on so-called Cyber Monday, when retailers expected the highest number of online transactions to take place ahead of Christmas - and prompted a flurry of card users to take to Twitter to vent their anger.
RBS said today that services had returned to normal, apologised to customers, and said it would cover those left out of pocket.
Customer services director Susan Allen told Sky News the problems had lasted for about three hours, and that, while the cause was still being investigated, it was known to be “completely unrelated” to high volumes of usage on the day.
Reports of the IT failure began to surface on Twitter at around 6.30pm.
In May, another glitch left RBS and NatWest customers using mobile apps unable to access their accounts online.
It followed a major fiasco in June last year which saw payments go awry, wages appear to go missing and home purchases and holidays interrupted - and cost the group £175 million in compensation.
Chief executive Stephen Hester announced that he would forgo his annual bonus at the time in the light of the problems. He was recently succeeded by Mr McEwan, who ran RBS’s retail arm from August 2012 to September 2013.
The new boss admitted last month that the bank still receives far too many complaints “often on issues that would never arise if our systems and processes were more effective”.
Trade union Unite, which represents RBS staff, called today for the bank to halt its cost-cutting programme - which has seen thousands of jobs axed and IT functions sent abroad - in the wake of the IT problems.
National officer Dominic Hook said: “It is unacceptable that the bank’s customers are once again facing inconvenience. Unite has grave concerns that staffing challenges are exacerbating the problems facing the bank.”