Fortnum & Mason, the luxury London department store, has still not delivered all the hampers customers ordered for Christmas after being floored by an IT glitch in December.

The company, famous for its luxury hampers costing up to £5,000, blamed the IT failure on the technical complications related to an IT systems upgrade and a three-fold increase in online transactions compared with the previous year.

A spokesperson for the company said: "Fortnum & Mason experienced severe issues with IT infrastructure that caused delays to some Christmas deliveries. This was not acceptable and we are working with our IT suppliers to ensure that we do not disappoint our customers again in the future."

Fortnum & Mason claimed that "the vast majority" of customers received their orders before Christmas, but did not confirm how many were still awaiting deliveries. The last order dates for Christmas for UK delivery was 19 December.

It also declined to reveal the identity of the IT suppliers.

To try and rectify the problems, the company said that it employed more than 100 customer service staff and increased the number of employees working overtime and in its warehouses.

"Everyone in the business, from the managing director [Beverley Aspinall] to the temporary Christmas staff, is working hard to rectify the problems.

"We will offer a full refund to anyone who did not receive their order and our customer service team is working through any outstanding issues on orders," the spokesperson said.

The Royal Mail is another UK company that has been unable to cope with the Christmas rush. Its online postage service on its Capgemini-managed website went down in December following a migration of online data to new servers, and problems with the site are ongoing in the New Year.

In contrast, retailers Next and John Lewis reported record online sales over the Christmas period, in Next's case, providing a much-needed lift to brick and mortar sales.