The latest IT fiasco at the Royal Bank of Scotland has added to the Financial Conduct Authority's workload under its ongoing investigation into the bank's faulty computer systems.

A recent computer failure left scores of RBS and NatWest customers without access to their bank accounts and other essential services on Cyber Monday, the biggest day of the year for online shopping.

Many customers could not access online or phone banking services or make card or PayPal payments. Others have reported not receiving welfare payments or being able to make council tax payments.

The FCA launched an enforcement proceeding following IT failures in June and July 2012, which left many RBS and NatWest customers without full access to their accounts for a week. The newest system failure will now form part of the existing probe.

"We are working with the firm to understand what has happened and help identify and fix the problem," said a spokesman from the FCA to IBTimes UK.

The FCA also said it would not be starting a fresh probe into to the latest IT glitches at RBS and declined to comment whether it would take any disciplinary action at the current time.

Since the crisis unravelled, RBS told affected customers they will be compensated for any losses. RBS said it has resolved the main problem, but issues remain for some customers in the fallout.

RBS Chief Executive Ross McEwan blamed a lack of investment in IT over the years for the outage.

"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," said McEwan.

"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."