The migration of millions of Alliance & Leicester customer records to Santander's banking system has left some customers unable to access their online bank accounts.
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Santander took over Alliance & Leicester in July 2008 and the bank recently started a phased migration of accounts for more than five million customers.
A Santander spokesman said although the vast majority of customers had a trouble-free upgrade to their banking service, "A small number of individuals have had temporary problems regarding specific internet banking transactions."
Santander was unable to indicate when full online services would be resumed and said it was dealing with customer queries. "We apologise for any inconvenience that has been caused to these customers and we are working to ensure these issues are rectified," said the bank.
The bank's business plan was to move all of its acquisitions - Abbey National, Bradford & Bingley and Alliance & Leicester - to its Partenon core banking system to cut costs. Customers of Santander's acquisitions will get the same Santander service by standardising IT across all outlets. Santander said it expects to complete the upgrade and rebranding of the Alliance & Leicester business by the end of 2010.
Services to Abbey customers were disrupted in late 2007 as it migrated to Partenon. The bank said only a small number of customers were affected. But staff said at the time that they were under pressure because of difficulties with the roll-out.