Barclays' online customers were told to try later
Barclays says it has fixed a technical fault that left some UK customers unable to withdraw cash from ATMs.
Its online and telephone banking services across the UK were not working throughout the afternoon.
The bank said that a hardware failure caused the issue, but services should gradually get back to normal.
A spokeswoman for Barclays, which has 3,500 UK cash machines and 15 million UK customers, apologised for the inconvenience caused.
The spokeswoman told the BBC that engineers replaced the faulty equipment - a "disc array" - in the afternoon and services should be completely rectified by the end of the day.
Barclays' online service was affected for longer than telephone banking, although the bank warned customers might face queues on phone calls after the fault was fixed at around 1700 BST. Some Barclaycard customers were also affected.
Generally, cash machines were affected in the south of England but customers in other areas of the UK were suffering problems if their details were stored electronically in certain areas.
BBC News website readers from London, Gosport, Farnham, Perth and mid-Glamorgan all reported being locked out of their Barclays accounts.
Rob Shaw, of Saltburn-by-the-Sea in North Yorkshire, said that his debit card was declined at the local supermarket when he tried to use chip-and-pin. He also tried to make a telephone transaction but was told to ring back in three hours.
"I appreciate that these things happen but it should not take so long to rectify and back-up systems should be in place," he said.
The Barclays spokeswoman said that the "vast majority" of chip-and-pin transactions should have been successful. Branches were also allowing customers to withdraw cash from the counters, although opening hours were not extended.
Customers using Barclays' debit cards in cash machines with other banks' branding were told services were unavailable when trying to withdraw cash.
A message, posted on the Barclays' website just after 1700 BST, said: "Technical problems this afternoon meant that you may not have been able to log-in to online Banking. We apologise for any inconvenience this may have caused."
It is not the first time the bank's customers have suffered such problems. In 2005, millions of customers were affected when 1,500 cash machines were affected by a computer glitch.
On 7 June, customers of the UK's biggest building society - the Nationwide - found they were unable to access their online accounts. The technical problem was fixed within a day.