The Financial Conduct Authority is investigating the Royal Bank of Scotland over IT failures at the bank last summer.
In a statement on its website, the FCA says it is looking into failures between June and July 2012 that saw customers unable to pay bills or access cash from their accounts.
A software upgrade saw RBS, NatWest and Ulster Bank customers face disruption to account services for two weeks. In August RBS agreed to pay out £125m in compensation to those affected.
The FCA handbook allows the regulator to report on investigations in “exceptional circumstances” and it considered the high level of public interest in RBS’ IT glitches to be meet the criteria. It will reach its conclusions in due course and will decide whether enforcement action should follow.
In February RBS was fined £390m worth of global fines after regulators found its traders had attempted to rig Libor.
An RBS spokesman says: “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.”