LONDON: Royal Bank of Scotland's systems crash that left more than 1 million British and Irish customers unable to withdraw cash or pay for goods on Monday has revived concerns about the resilience of the state-backed bank's technology.

The three-hour outage on one of the busiest shopping nights of the year could cost RBS millions of pounds in compensation and comes as Britain's regulator is threatening to fine the bank for the collapse of its payment systems last year.

The regulator has been scrutinizing the resilience of banks' technology amid concerns that outdated systems and a lack of investment could cause more crashes.

From 1830-2130 GMT on Monday, RBS's cash machines didn't work and many customers trying to pay for goods with debit cards at supermarkets and petrol stations, buy goods online or use online or mobile banking were unable to complete transactions. The bank said on Tuesday the problem had been fixed.

"All our services are now back working normally. We would like to apologize to our customers. If anyone has been left out of pocket as a result of these systems problems, we will put this right," a spokeswoman said.

About 250,000 people an hour would typically use RBS's cash machines on a Monday night, and tens of thousands more customers would have used the other affected parts at its RBS, NatWest and Ulster operations. RBS has 24 million customers in the UK.

Twitter lit up with complaints from angry customers.

"RBS a joke of a bank. Card declined last night and almost 1,000 pounds vanished from balance this morning! What is going on?" tweeted David MacLeod from Edinburgh.

Some people tweeted on Tuesday they were still experiencing problems and accounts were showing incorrect balances.

RBS is 82 percent owned by the UK government and millions of its customers were affected in June 2012 by problems with online banking and payments after a software upgrade went wrong.

That cost the bank 175 million pounds ($286 million) in compensation for customers and extra payments to staff after the bank opened branches for longer in response. Stephen Hester, chief executive at the time, waived his 2012 bonus following the problem. Britain's financial watchdog is still investigating and could fine the bank.

The latest crash occurred on so-called Cyber Monday, one of the busiest days for online shopping before Christmas. RBS gave no details on the cause of the problem, but it is not expected to be due to the volume of transactions.

The bank's new chief executive Ross McEwan has vowed to improve customer service and has said technology in British banking lags behind Australia, where he previously worked. He has pledged to spend 700 million pounds in the next three years on UK branches, with much of that earmarked for improving systems.

RBS's former CEO Fred Goodwin has been blamed for under-investing in technology and for not building robust enough systems following its takeover of NatWest in 2000.

Andy Haldane, director for financial stability at the Bank of England, told lawmakers last year that banks needed to transform their IT because they had not invested enough during the boom years. Haldane estimated 70-80 percent of big banks' IT spending was on maintaining legacy systems rather than investing in improvements.