If Prince William's girlfriend joins the royal family for Christmas this year, she will be making history in the palace.

Britain's Queen Elizabeth has reportedly invited Prince William's girlfriend to Christmas lunch with the family.

Kate Middleton, 24, is the first unmarried partner to be invited to the festive celebrations at the royal estate of Sandringham, in Norfolk.

A source told Britain's Mail on Sunday newspaper, "Kate has been in invited to Sandringham for Christmas lunch, but she has yet to accept. It is incredibly prestigious, so she is unlikely to turn down such an important invitation."

Kate - who has been dating William, 24, for three years after they met at St. Andrews University - is said to be excited about spending Christmas with her royal boyfriend, but is worried about upsetting her own family if she accepts the invite.

A friend revealed, "She is incredibly close to her parents and her brother and sister, but Kate knows that, if she and William stay together there will have to be compromises. It is a case of each of them balancing Christmases between their two families - and at the moment she is leaning towards Sandringham."

The friend added, "She knows that it is a great honor to be asked and that William would never not be present at the Christmas lunch."

Kate's extended festive invitation is bound to fuel speculation that she and William will soon announce their engagement.

Last year, the brunette was only invited to join the royal family for their traditional Boxing Day shoot.

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