Lloyds Banking Group has apologised after customers of Lloyds, Halifax, Bank of Scotland and TSB were unable to to access ATMs and credit cards.
Customers were unable to withdraw withdraw cash from thousands of ATMs or pay for services using credit or debit cards on Sunday, prompting an apolgy from the bank, which claims that all problems have now been resolved.
"We apologise that earlier today, between 3pm and 6pm, some customers were unable to complete their debit card transactions," the bank said in a statement.
"Although the majority of transactions were unaffected, we are very sorry for the inconvenience that this will have caused.
"At the same time, some customers encountered problems at approximately half of our 7,000 ATMs. This was resolved by 7.30pm, and all of our ATMs are now working."
TSB CEO Peter Pester denied that the problems were related to IT outsourcing, tweeting that the "issue was caused by a HP server failing here in the UK".
TSB, which has approximately five million UK customers, is in the process of being split from Lloyds Banking Group, and still relies on the Group's IT systems.
Last year Lloyds Banking Group confirmed to ComputerworldUK that 190 back office IT jobs would be outsourced to India as part of cost-saving measures.
Lloyds Banking Group confirmed that the problems were hardware related. The Group did not rule out whether any of its software systems were to blame for the fault, or that system upgrades were taking place prior to the outage.
UK banks have experienced a number of problems in recent years, largely due to legacy IT systems. In 2012 a botched software upgrade created widespread disrupton to customers of Royal Bank of Scotland, Natwest and Ulster Bank, with the banking group experiencing a recurrence of problems in late 2013 due to an unspecified IT fault. The RBS problems prompted an investigation from banking watchdog the Financial Conduct Authority (FCA).
Last year Lloyds Banking Group blamed a data centre hardware malfunction for an outage which marred the re-launch of TSB last year, as the bank's website crashed, leaving customers unable to access online account services.