The Financial Conduct Authority (FCA) has started an enforcement investigation which could lead to a financial penalty for the bank.
For more than a week in June and July customers could not access their accounts or make or receive transfers.
The fault was blamed on an IT operative in India accidentally deleting vital information.
“The Financial Conduct Authority 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 said in a statement.
“The FCA will reach its conclusions in due course and will decide whether or not enforcement action should follow that investigation.”
RBS pledged to help the investigation.
“Last summer's IT failure was unacceptable. We have already made significant improvements and over the next three years will invest hundreds of millions of pounds in our systems,” it said in a statement.