Barclays customers were hit by technical problems affecting the bank's network of cash machines today.
Sources said "hardware errors" impacted on various areas of the business, including its online banking customers.
It is thought that Barclaycard customers could also be affected. Barclays was unavailable to comment.
Customers attempting to log on to online accounts were informed that Barclays was "currently experiencing technical problems" in online banking and told to try again later.
Barclays' retail banking arm supplies more than 15 million individual and business customers across the UK with current and savings accounts, mortgages, loans, credit cards and insurance.
The division made pre-tax profits of £1.37 billion in 2008, out of group profits of more than £6 billion.
A Barclays spokeswoman said the bank was suffering from a "hardware failure" which was affecting some customer systems.
"We are still trying to find out what the full impact is. We apologise to anybody who is inconvenienced," she said.
Barclays was also trying to establish whether its entire nationwide network of almost 3,500 cash machines was affected by the problems.