Prince William's girlfriend Kate Middleton has been given an unprecedented invite to spend Christmas Day with the British royal family, The Mail on Sunday newspaper reported.

For royal watchers, Elizabeth's breaking her strict rule on no boyfriends or girlfriends at the Christmas table can mean only one thing: A royal wedding is in the cards.

Last year Middleton was only invited to spend the following day, Boxing Day, with the royals.

Middleton now has to decide whether to spend December 25 with her family or William and the queen, The Mail on Sunday said.

A source told the tabloid: "Kate has been invited to Sandringham, Queen Elizabeth II's private estate for Christmas lunch, but she has yet to accept. It is incredibly prestigious, so she is unlikely to turn down such an important invitation.

A spokesman for Clarence House, William's official residence, said: "Christmas plans are being discussed, but we do not discuss royal guests."

William met Middleton at St Andrews University in Scotland, where he earned a master of arts degree in geography.

They have been together for four years and lived together during their student days. Their relationship is kept as closely guarded as possible.

British retailer Woolworths has produced 100,000 commemorative souvenirs ready to be deployed to stores if William and Middleton get engaged.