Lady Williams, 52, was confronted outside her apartment block in South Kensington, west London by two muggers, one carrying a sheath-knife. They forced her to hand over rings and the keys to her BMW estate car.
The attack is the latest robbery of celebrities in London, including the actress Britt Ekland, the chairman of Carlton Communications, Michael Green, the model Caprice and Bernie Ecclestone, the Formula One boss. Detectives are investigating whether the attacks have been carried out by a single gang or individual muggers.
In the most recent case, which happened last Thursday, although details have only just emerged, the muggers escaped in Lady Williams's car, leaving her shocked but not injured.
Scotland Yard is seeking two men. The suspect carrying the knife was described as black, aged about 19 and wearing a blue baseball cap and blue sweatshirt; the other was black, in his mid-20s and stockily built.