Tesco Bank has fixed a computer glitch that locked 300,000 of its customers from their accounts for two days.
The banking arm of the UK supermarket chain resumed normals service early this morning.
Up to one-in-twenty of the bank's seven million-plus customers had been unable to access their accounts since Tuesday.
Tesco Bank runs its operations on a core platform from Fiserv. It claims that up to 86% of transactions are conducted online.
The bank has yet to offer an explanation for the cause of the breakdown.