Kate Middleton sits in the stands as her boyfriend, Prince William, takes part in the Sovereign's Parade at The Royal Military Academy in Camberley on 15 December 2006.

The girlfriend of Prince William has reportedly turned down a rare invitation to join the royal family for Christmas.

Kate Middleton, 24, has said she will only join them if she and the future king get married, the Mail on Sunday reported.

Media speculation² that the couple, together for four years since meeting at St Andrews University in Scotland, could be about to get engaged was fuelled when Middleton appeared Friday at William's graduation from Sandhurst, the British military academy.

But despite their closeness, she has decided³ to spend the festive⁴ season with her own family after the recent deaths of her two grandmothers, the Mail reported.

"Kate has said she won't be going to Sandringham (the Queen's country home) until she's a part of the family," the paper quoted a friend of Middleton as saying.

"She has made it quite clear that she will only be at a royal Christmas if she and William get married and that's not yet on the cards.

"We all know it's going to happen but it's not going to happen this Christmas."

The friend reportedly added that Middleton and William keep a wall chart with a tally⁵ of how many times newspapers predict they will get married.