Tesco Bank, owned by the UK’s biggest supermarket chain, which has more than five million customers, apologised for the shutdown which lasted about four hours from 9.30am, according to the BBC News website.
A spokeswoman for the bank told the BBC: "We apologise to customers who were unable to access online and mobile banking earlier today. These services are now working as normal and we would like to thank customers for their patience."
In November 2016, Tesco Bank was hit by a "sophisticated” cyber-attack which saw money taken from 20,000 accounts.
The latest disruption comes after banking rival TSB last month saw up to 1.9mln customers unable to access its banking services due to an outage caused by a botched IT migration.