The decision to invite Mrs Parker Bowles, the woman the Princess blamed for the failure of her marriage, to Spencer House has been branded "insensitive".

Spencer House's current tenant, financier Lord Rothschild, has invited the couple to a party on Thursday to celebrate the Prince's 50th birthday.

A private music recital and dinner is planned.

Public support for the relationship has grown

The pair have been seen together at a number of functions recently and have gained public support for their relationship.

But the champagne celebration could prove a setback for the Prince's partner after Spencer family sources raised concerns that the event is being held at a location closely linked with Diana's family name.

Private function

St James's Palace said the dinner was a private, unofficial event, being held at the invitation of the Duke and Duchess of Devonshire and Lord Rothschild, who holds a 120-year lease on the house.

Sandy Henney, spokeswoman for the Prince of Wales, said: "This is a private engagement at the invitation of Lord Rothschild.

"It is an unofficial engagement and to suggest that it forms some part of the Prince's official duties is completely untrue."

The 18th-century mansion, overlooking Green Park and within a few hundred yards of Buckingham Palace, has been extensively renovated by Lord Rothschild, who has had control of the property as tenant for the last 10 years.

Diana blamed Camilla for the failure of her mariage

But ownership is retained by the Spencers. Diana's sisters, Lady Jane Fellowes and Lady Sarah McCorquodale, and her brother Earl Spencer each hold a quarter share.

The Princess's holding in the property passed to Princes William, 16, and Harry, 14, on her death last year.

Members of the Spencer family are reported to be distressed at the visit by Mrs Parker Bowles.

A family source said: "The house is leased to a private individual but with it being called so prominently Spencer House you would have thought they would see the insensitivity of this."