Tradition dictates that if Christmas decorations are not tidied away by today, households will be plagued by bad luck for the rest of year.

The centuries-old Twelfth Night tradition could leave hundreds of the nation's wealthiest in a spot of bother as many are still awaiting Christmas hampers costing up to £5,000 from Fortnum & Mason.

The world-famous Piccadilly emporium has been forced to admit that it still has not delivered orders placed weeks before Christmas.

Fortnum, which traces its roots back to 1707, said it has still not been able to deliver all of its orders for festive treats, including its £40 Queen Elizabeth Christmas pudding with gold-leaf almonds.

The Queen's favourite grocer said it was still reeling from an IT glitch that brought down its electronic delivery system before Christmas. Despite drafting in 100 extra customer service staff, including managing director Beverley Aspinall, the store is still faced with a significant backlog almost two weeks after the big day.

A spokesman for the company, which has previously managed to deliver a hamper to an expedition up Mount Everest, said the "vast majority of customers received their orders before Christmas" but admitted there many are still awaiting their box of treats.

"Fortnum & Mason experienced severe issues with IT infrastructure that caused delays to some Christmas deliveries," the spokesman said. "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.

"We are very sorry that this situation occurred. We have been doing everything in our power to rectify it for our customers, so that we can deliver the excellence of service that they expect. 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 company would not say how many people were still awaiting delivery.

The long delay is likely to embarrass the company, which has boasted Queen Victoria, Oscar Wilde, Elizabeth Taylor and Frank Sinatra among its celebrity clientele.

On its website, the Piccadilly store states that: "It's at Christmas that the Fortnum's hamper really comes into its own. After all, what could be more satisfying than to have Christmas delivered to your door? Our Christmas hampers are designed to provide just that, as each one contains all the essentials and luxuries that one needs for a proper, slap-up feast.

"A Fortnum's hamper is the ultimate indulgence at Christmas – filled with edible and drinkable delights and luxuries, it is quite the best thing to find on your doorstep on a snowy morning.

"When the Fortnum's Christmas hampers are put together each year, it is done with two things in mind. Everything inside must be the finest of its kind; and there must be a Fortnum's hamper for everyone."

Fortnum sells 40 different hampers including a £5,000 Imperial Hamper packed with Beluga caviar, foie gras truffles, cognac butter, a magnum of Cristal 2002 and a bottle of 32-year-old whisky.