Kate turns 30 on Monday but is not expected to have a lavish party to mark the occasion.
She reaches the milestone after a year that has seen her marry royalty, embark on her first official overseas tour and take on several patronages.
There had been reports that Prince Harry was planning a big bash for his sister-in-law.
But a St James’s Palace spokesman said: “Whatever is planned will be low-key and private.”
It is thought Kate will spend her birthday with William and close family and friends.
The day before her celebrations the duke and duchess will attend the UK premiere of War Horse in London’s Leicester Square, where the world’s media will be waiting. William, an RAF search and rescue helicopter co-pilot, is rostered to work next week from his base at RAF Valley in Anglesey. So this may have also influenced the couple’s decision to have a quiet celebration.
Turning 30 will cap a life-changing 12 months for the new royal, who married her fiance on 29 April last year in a glittering Westminster Abbey ceremony in front of a television audience of billions. .
Earlier this week it was announced the duchess had agreed to support four organisations, two as patron – Action on Addiction and the National Portrait Gallery – and East Anglia’s Children’s Hospices and The Art Room as royal patron. The duchess will also become a volunteer with the Scout Association.