THE ROYAL Family and the Spencer family are to be united for the first time since their public rift at the funeral of Diana, Princess of Wales six years ago.

Both families will attend Prince William's 21st birthday party at Windsor Castle next month. The Prince, who comes of age on June 21, is determined to hold an "inclusive" celebration which his father, the Prince of Wales, and his grandmother, the Queen, will attend along with Earl Spencer, his mother's brother, and his mother's two sisters. Prince William, who senior royal aides say has wisdom and common sense beyond his years, will be seen as a peacemaker. Palace officials hope that the party, on the evening of his birthday, will mark the end of years of private and, at times, open hostility between the two families.

"Prince William has never fallen out with anybody and he wants all his close family and friends to be with him when he celebrates his birthday," a senior royal aide said. It had been widely assumed that the Spencers would host a separate birthday celebration for Prince William. Tensions between the Windsors and the Spencers began when the marriage of the Prince and Princess of Wales disintegrated in the 1980s.

The ill feeling was exacerbated when Earl Spencer delivered his funeral tribute to his sister at Westminster Abbey in 1997 when he pledged that William and Harry's "blood family" would ensure the Princes would not be "immersed in duty and tradition". Only last week, the Earl made a veiled attack on the Royal Family in a BBC2 programme, Althorp: After Diana. "There are a lot of people who'd like Diana forgotten," he said. The Earl and Prince Charles are believed to have met just once since the funeral: they fleetingly recognised each other at a reception in Cape Town.

There has been further friction between the two families over the collapsed Old Bailey theft trial of Paul Burrell, the former butler to Diana, Princess of Wales, who died in a car crash in Paris in 1997. For a long time, the Prince of Wales did not think that Mr Burrell should be prosecuted, whereas the Spencer family, particulary Lady Sarah McCorquodale, the Princess's sister, were in favour of charges.

Prince Charles and the Queen were consulted over the guest list for the party, but it was ultimately down to Prince William, who is a studying for a degree in history of art at the University of St Andrews. The party has an "Out of Africa" theme.

Camilla Parker Bowles, Prince Charles's companion, has been invited to the party and has accepted. In past years, the Queen has been reluctant to be at the same event as Mrs Parker Bowles.