On the morning of what was promised to be a 'seamless transition' for the Lloyds and TSB bank split, 'unexpected high volumes of traffic' caused the website to crash, said a TSB spokesperson.
The crash left customers unable to access their accounts online, as the two banks split off in a rebrand forced by the EU to comply with competition rules on the high street.
A surge of traffic to the website and those of other banks in the Lloyds Group caused the problems, undermining the promise of the chief executive, António Horta-Osório, of a "seamless" transition as TSB was relaunched.
Customers at Lloyds and Halifax were also affected by the glitch, which left some unable to even load the websites and others unable to log in. Only the Bank of Scotland website was unaffected.
A spokesperson for the group said: "We are experiencing an issue with our internet banking service this morning, which has affected the ability of some customers to log on successfully.
"We are working to resolve this as quickly as we can and we apologise to customers for the inconvenience this will have caused. Our branches, telephone banking and cashpoint facilities have not been affected in any way."
On Saturday night the websites were closed as final technical work was done ahead of Monday's official return to the high street of the TSB brand, which came with the opening of the first rebranded stores. Other branches will follow in the first three days of the week.
Michael Allen, director of APM at Compuware said:
"Given the uncertainty that the split has caused for customers, for them to then not be able to get into their accounts this morning will have created unnecessary stress for thousands, if not millions, of customers.
"Banks need to manage the risks associated with technology performance in a more proactive way, with early warning systems that can alert them to any potential problems before they create chaos for their customers.
"For example, monitoring these services from an end-user's perspective to see how they perform in the real world will identify any weak spots and inefficiencies and enable them to be rectified ahead of demand hitting, rather than after the complaints have begun to pour in."
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