The store, which is undergoing the biggest overhaul in its 300-year history, has completed the transformation of its Fresh Food Hall and the construction of a huge atrium rising four storeys through the centre of the building.
While the stainless steel banisters, grey stone paving and shiny metal cabinets give the space a tasteful modern appearance, they are likely to upset traditionalists.
As is the new wine bar called 1707, the date the store was founded, which was designed by David Collins, the man responsible for the decor in some of the country's hippest restaurants and the London home of Madonna.
Gone is the dated chintz, to be replaced by a black marble bar, hand blown glass lamps and a white mosaic tiled floor.
The Piccadilly store, whose customers have included Charles Dickens, Winston Churchill and Jacqueline Kennedy, is also opening a take-away service selling coffee and sandwiches.
But being Fortnum's fillings include foie gras, the finest English cheddar and Welsh rarebit.
Modern curiosity cabinets, placed around the store, will contain truffles, foie gras and caviar, all kept at precisely the right temperature. A meat counter, with hand-carved and aged cuts, a fish counter and a Georgian market of fresh produce complete the epicurean offerings, along with the traditional store cupboard goods such as Gentleman's Relish, 16 types of mustard and olive oils.
Beverley Aspinall, the managing director, who oversaw the transformation of Peter Jones, the Chelsea department store, reassured customers that the essence of the store would remain. "This is the first stage and the most complicated stage," she said. "It is modern but we have used only classic materials of oak, marble and steel.
We didn't want a pastiche.
"I liken the store to a Georgian house that has been kept up to date. You wouldn't expect the owners of a Georgian house to have the original kitchen and plumbing.
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The newly refurbished floor will be opened on Monday and then work will start on the other three storeys and dining rooms.
Despite its traditional image, Fortnum and Mason, which was set up by Hugh Mason, a shopkeeper, and William Fortnum, a footman to Queen Anne, has been famed for its innovation.
When Dickens finished a book he would reward himself with a hamper from the food hall and Edward VIII had kippers and marmalade shipped to Paris for his breakfast on the day that he married Wallis Simpson.
