The Financial Conduct Authority (FCA) has announced an investigation into IT failures which caused widespread service disruption at RBS last year.

Millions of customers at RBS, Natwest and Ulster Bank were unable to access accounts due to technical problems as a result of software upgrade. It took more than a month to return banking systems to normal.

The FCA said in a statement that it “has started to conduct an enforcement investigation into the IT failures at RBS which affected the bank’s customers in June and July 2012”.

“The FCA will reach its conclusions in due course and will decide whether or not enforcement action should follow that investigation.”

Following the outage RBS, which is 80% owned by the state, committed to replacing systems, with a £80 million overhaul of mainframe systems. The bank estimated that the IT failure cost £175 million, including charges and compensation payments for customers.

It was revealed that the problems were initially caused by a botched upgrade to batch processing software CA 7 from CA Technologies.

RBS chief executive Stephen Hester has been criticised for not investing in upgrading the bank’s legacy systems after he took control of the sprawling mainframe estate in 2009.

Commenting on the FCA investigation, an RBS spokesperson said that improvements had been made to its IT systems.

"Last summer’s IT failure was unacceptable. We have already made significant improvements and over the next three years will invest hundreds of millions in our systems.

"We will be working closely with our regulators in the UK and the Republic of Ireland. Our customers deserve a service they can rely on 100 per cent of the time and that’s what we want to provide."

Following the high-profile IT problems in 2012, RBS experienced failures again in March. Natwest customers were left unable to make online banking transactions, cash withdrawals or debit card payments for a number of hours after system problems relating to a hardware failure.