Co-operative Bank customers have been unable to use their debit cards for some transactions, following a weekend software upgrade.
The problems emerged on Sunday, and were fixed yesterday, the bank said. It did not immediately comment on the system affected.
The problems specifically affected foreign transactions, including gambling sites, the bank said. Around 5,000 of the bank's two million customers are understood to have been affected.
"We have now resolved the issues identified and have been proactively working with affected customers," the bank said in a statement. "We would like to apologise for any inconvenience caused."
In September, Boots IT director Andy Haywood was appointed the new Co-operative Group chief information officer. He begins in January.