The party is being held to celebrate a number of royal birthdays including Prince William's 18th, on Sunday, and the Queen Mother's 100th.
The 40-year-old mother of Prince Andrew's daughters Beatrice, 11, and Eugenie, 10, will join guests at Windsor Castle on Sunday 21 June for a royal party.
The party is being held to celebrate a number of royal birthdays including Prince William's 18th, on Sunday, and the Queen Mother's 100th.
The invitation is being seen as an attempt by the Queen to heal a rift with the Duchess, who has not been in royal favour since the break-up of her marriage with Prince Andrew.
Royal opinions about her did not improve when tabloid newspapers took pictures of the Duchess having her toes sucked by former financial advisor, John Bryan.
She has also been criticised for portraying the royal family in a bad light on several occasions, including appearing in American television adverts.
'Family unity'
Although she joined royal mourners at the funeral of her good friend Diana, Princess of Wales, in 1997, the door has remained firmly closed since then.
She has not been allowed to celebrate Christmas with the rest of the family, although the Queen has remained on good terms with her.
Last year, the Duchess was excluded from the Prince Edward's marriage to Sophie Rhys-Jones, although Prince Andrew and their children did attend.
Prince Philip was reportedly furious with the Duchess when she moved back in with Andrew to their former marital home, and the couple are living together as a family although they are still divorced.
Last year the Duchess and Andrew refused to rule in or rule out re-marriage, although most experts believe that is unlikely to happen.
On Tuesday the Duchess was visiting a hospital in Birmingham to open a specialist cancer care unit for young people.
She declined to comment on the invitation, but said: "It is a wonderful year for Her Majesty. It is wonderful to see family unity at a time to honour such a great lady."
Prince Charles' long time companion Camilla Parker Bowles has not been invited to the 21 June party, although the queen recently met her for the first time in eight years, at Prince Charles' Highgrove home.