Prince Charles has been accused of "cronyism" after the Duchy of Cornwall employed his sister-in-law as an interior designer.
Annabel Elliot renovated three Duchy properties
Annabel Elliot, the Duchess of Cornwall's sister, worked on three holiday cottages on Charles's estate.
Labour MP Ian Davidson said it was unacceptable and it raised questions about how Charles runs the Duchy.
A Clarence House spokeswoman said Mrs Elliot was a professional and had been employed "for her expertise".
Mr Davidson said: "He seems to be giving it to this person just because she's the sister of his wife.
"It's nepotism and cronyism. It must be a tremendous boost in publicity having a contract with the Duchy and the prince.
"It raises questions about how Charles runs the Duchy."
The Public Accounts Committee, of which Mr Davidson is a member, publishes its report into the Duchy of Cornwall and Duchy of Lancaster on Thursday.
DUCHY OF CORNWALL
Created 1337 by Edward III for his son, Prince Edward
Provides income for the heir apparent
About 140,000 acres in 25 counties, including Oval cricket ground
Last year the Duchy generated £13.3m
Mrs Elliot renovated the 17th Century Guard House and another cottage on the Isles of Scilly and 200-year-old Restormel Cottage in Lostwithiel, Cornwall.
A Clarence House spokeswoman, said: "Annabel Elliot is a professional designer. She has been employed for her expertise. We're delighted with the results.
"We've had excellent feedback and excellent bookings as a result."
She added: "At the end of the day, it (the money) comes from the revenue from which the prince gets his private income. It's not required to go to a tendering process."