IT director Maggie Miller said the improvements made by Accenture and its internal IT team between Christmas 2001 and 2003 were adding the equivalent of "tens of millions of pounds" in extra sales at one of its busiest times of year.
All systems are tested at Christmas peak capacity before they go live and after every upgrade, she said.
"The improvement is systemic. It is not just focused on the Christmas period, it is making sure all our systems are designed for reliability and resilience."
Sainsbury's locks down its systems weeks before Christmas to ensure they are not put at risk by upgrades. A "soft lock-down," in which some essential changes can be made, is followed by a "hard lock down" to freeze all systems.
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