Brand Finance's British Luxury 2018 survey, Kate wearing a product increases the desirability among 38 per cent of shoppers.
The report found Meghan improves the desirability by 35 per cent, with Harry increasing a brand's prospects by 32 per cent.
Commenting on the report, Alex Haigh, director of Brand Finance, said: "The monarchy is one of the most defining features of Brand Britain and the royal impact on the desirability of British luxury brands cannot be overstated.
"Endorsement by any royal, but particularly the Duchesses, is like the mythical golden touch and can transform a brand’s performance overnight."