Amanda Wakeley, whose creations are worn by the Duchesses of Sussex and Cambridge, as well as actresses Kate Winslet and Angelina Jolie, was set upon in the Chelsea Harbour district of west London on Wednesday morning, according to her partner and financial PR adviser, Hugh Morrison.
Ms Wakeley was investigating her flat tyres when the assailants pounced, threatening her with an axe, Mr Morrison said.
The thieves took off with the 57-year-old's £25,000 Rolex watch, leaving her shocked but unhurt, according to Mr Morrison.
"The police have been amazing, but say that this happens five to six times every day in [Kensington and Chelsea] and have asked us to share this," Mr Morrison said in a social media post.
"There were a lot of witnesses and police feel that the culprits will be caught. In the meantime, Amanda just wants to share her experience and hopes that everyone stays safe."
Mr Morrison shared a photograph of the alleged muggers, one of whom wore a blue helmet.
Statistics show moped crimes have halved in London since the Metropolitan Police launched Operation Venice to counteract the problem. There were 23,986 crimes in which mopeds, scooters or motorbikes were used to carry out an offence in 2017-18, but this dropped to 11,390 in 2018-19, The Times newspaper reported.