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Princess-to-be Kate Middleton has tendered her letter of resignation -- to Mum and Dad!

With her royal wedding three months away (!) Middleton, 29, has quit her job at Party Pieces, her family's online party supply business, a Palace spokesperson confirmed to

Prince William's common-born fiancee left her position as project manager in order to "concentrate full-time on preparing to become a member of the royal family," the spokesperson said.

Middleton had served as Party Pieces' website designer and photographer before stepping down Friday.

Planning continues apace for Middleton and her prince, 28, whose big day is April 29. The biggest challenge: trimming down the guest list for the reception down to a mere 100 (William's dad Prince Charles gets 100 of his own). "William and Kate admit this has been the most stressful part of the wedding," an insider recently told Us Weekly.