Prince William's wife was seen arriving at the luxurious hairdressers she usually visits before the SportsBall on Thursday and was quickly escorted from her car to the VIP club suite, located on the third floor.
"She looked amazing," said one onlooker. "She was dressed in a tartan cape and was made up.
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"Her hair looked fabulous," they added. "Really big and glossy, and it was flowing around her shoulders."
The Duchess had a private session with one of the spa's leading colour technicians for several hours.
Staff at the Rossano Ferretti Hair Spa were reportedly told that a VIP was in the building and that the lifts were out of use to ensure that the Duchess was not interrupted during her visit.
Kate was first spotted at going to Ferretti's hair spa, which is also frequented by Carole Middleton, Pippa Middleton and members of the Saudi royal family, when she was pregnant with Prince George.
In June the Duchess, whose hairstyle is one of the most copied around the world, was photographed at the Mayfair spa, rather than Richard Ward's salon, where she was usually seen.
The salon, which is located in a chic townhouse, is part of a chain of hairdressers which can be found around the world, including salons in Pairs, LA, New York, Miami, Madrid, Rome and Venice.