Some Alliance & Leicester customers have been unable to carry out certain online transactions thanks to a systems glitch caused by the transfer of accounts to the Santander IT platform.
The Spanish bank acquired A&L in 2008 but only received court approval to transfer its assets to Santander UK in May, prompting a phased migration of accounts for over five million customers to its Partenon technology platform.
This will give customers access to other Santander features and all the bank's 1300 branches, acquired through its Abbey National and Bradford & Bingley buys. This network is set to expand further after a recent deal struck to buy 318 branches from RBS.
A spokesperson says "the vast majority" of customers have had a trouble-free "upgrade" so far during the process, which is expected to be completed by the end of the year.
"However a small number of individuals have had temporary problems regarding specific internet banking transactions," says the spokesperson, advising those affected to call or visit the site.
No detail has been provided on the exact cause of the problem, how many customers have been affected or when it is likely to be fixed.