Diana stipulated in her will that William was to become eligible to inherit half of her estate - which is believed to have grown to around £20 million since her death in 1997 - on his milestone birthday.
But despite his huge windfall, William won't be throwing an extravagant party tonight.
He's said to have left for work as a search and rescue pilot in Gloucestershire, where he's currently taking part in a week-long written training course.
Kate Middleton won't let the big day pass without celebration, though - she and sister Pippa and mum Carole have been planning a surprise bash for Wills.
'She wants to make a roast dinner for 25 of his closest friends,' a source tells us.
'After dinner, she wants to kick off the party with a murder mystery game, which she knows he'll enjoy.
'Although there'll be a fair few of his friends attending, it'll still be a
romantic evening for them as he'll know that this is her way of showing
him how much she loves him.'
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Anna Duff
