Natwest and Sainsbury's Bank customers were unable to make payments due to separate IT glitches yesterday.
Natwest - which is partly state-owned - issued an apology to customers after a system failure prevented payments and transfers via online banking.
The incident lasted from approximately 1.30pm to 6pm yesterday, with around 12,000 payments missed per hour.
"During this time customers were able to process payments using telephone banking, our mobile app or by visiting a branch," a spokesperson said.
"We apologise to customers for the inconvenience this caused."
Systems have now been returned to normal, and the cause of the incident is being investigated.
The outage comes not long after Natwest's parent company, Royal Bank of Scotland, received a £56 million fine from regulators following a major outage in 2012 which affected millions of customers and lasted weeks.
The bank claims to have made efforts to improve the resilience of its systems, including separating batch processing systems, upgrading payments infrastructure, and increasing mobile banking infrastructure capacity.
In a separate incident, Sainsbury's Bank customers were unable to make debit and credit card payments and access savings due to an unspecified glitch on Tuesday. The challenger bank, which has approximately 1.5 million customers, restored all systems by 6.30pm.
A Sainsbury's spokesperson said: "We've reassured [customers] that the problem has been fixed and apologised for any inconvenience caused."
Formerly part of Lloyds Banking Group, Sainbury's Bank announced in 2013 that it would outsource its entire IT infrastructure to service provider FIS, as part of a three-and-a-half year process costing £90 million.
Recurrences of systems failures among UK banks in recent years have drawn the attention of regulators, with the Financial Conduct Authority continuing to investigate the resilience of key technology infrastructure.
Sign up for CIO Asia eNewsletters.