The memorial cost about �3m

She will be joined by Princes William and Harry, the Prince of Wales and the Duke of Edinburgh at the unveiling in Hyde Park, central London, on 6 July.

Earl Spencer, the Princess' brother, and other representatives of her family are also set to attend.

The �3m creation is being opened nearly seven years after Diana died in a car crash in Paris.

"This is no ordinary memorial," Culture Secretary Tessa Jowell said in a statement.

'Enjoyed by millions'

"Like the Princess herself, the fountain is open and approachable,

"I hope that it will be enjoyed by the millions of different people who come to visit it in the future and will be embraced by all as a permanent celebration of Diana, Princess of Wales."

The unveiling will be the first joint public appearance for the Royal Family and the Spencers since Diana's funeral in September 1997.

Children will be able to paddle in part of the fountain

Work on the fountain at the side of the Serpentine began last year, after a design by American architect Kathryn Gustafson was chosen by a memorial committee.

Based on a large oval stone ring, water pours into the structure from the top of a hill at the Serpentine Bridge, before running in two directions.

In one area it forms a tight, fast-flowing torrent, creating water jets, while in the opposite direction water forms a dish that "rocks and rolls" through a gentle incline.

Water from both sides collects in a dish shaped pool which will be shallow enough for children to paddle in.

Construction of the 260ft by 150ft fountain was beset by delays, and Diana's mother Frances Shand Kydd, who died last week, criticised the design for a "lack of grandeur".