The 25-year-old royal is whisking the 'Ted' star and several of her closest friends to Cannes, France, in May for a fun-filled celebration ahead of her tying the knot with her beau Ashton Kutcher.
A source told the Daily Star newspaper: ''Beatrice seems almost as excited about the engagement as the couple themselves.
''Like all Mila's inner circle, she's known marriage has been on the cards for some time. But now things are official she wants to crack on with the celebrations. Bea told Mila she wants to take her for a pre- wedding shower in Cannes along with six or seven of their mutual friends.''
However, Beatrice - who is the granddaughter of Queen Elizabeth - has promised it will be a night to remember as she has a few surprises up her sleeve to get the girls into the party spirit.
The source explained: ''Bea has promised there will be a few surprises. And Mila knows better than to assume just because her friend is royal she'd hold back on the party plans.''
Ashton, 36, and Mila, 30, have developed a close bond with the princess and her boyfriend Dave Clark after they were spotted on a series of nights out in London last summer.
Meanwhile, the loved-up couple, who have been dating for two years, sparked speculation they had got engaged earlier this month after the brunette beauty was spotted wearing a diamond sparkler on her ring finger.
His new album Underneath It All is out now.