Royal sources have played down speculation the prince is preparing to propose to his long-time girlfriend later this year, saying the couple may not tie the knot until 2010.

William and Kate sparked a flurry of excitement on Saturday as they were photographed at a friend's wedding in Austria.

However, a senior royal household source has played down the chances of Kate and William marrying soon. ''There is a lot of planning involved in a royal engagement,'' the source said.

''A number of high-profile people have to be informed formally, and the couple have to be introduced to the prime minister of the day, among others. All this takes time.

''As far as I am aware, I cannot see there being an engagement between the prince and Miss Middleton for at least 18 months.''

Meanwhile, William is reportedly preparing to join his younger brother, Prince Harry, on a charity motorbike ride through Africa to raise money for orphans and Aids victims. AAP