Allison Cox pleaded guilty to one charge of administering poison to Jacqueline Gold "with intent to injure, aggrieve or annoy." Prosecutors dropped two other charges of attempted poisoning with sugar and salt.
The hearing was the first time prosecutors had revealed a motive for the poisoning, which experts said would have been unlikely to cause much harm to Gold.
Cox, 33, was employed to look after Gold's daughter Scarlett when the poisoning took place last October.
Prosecutor Rachel Davies said Cox tampered with the food because of "some animosity from the defendant towards the cook. She put it into the food to cause the cook to be questioned about it."
The plot unraveled after Gold criticized the cook about the odd-tasting food. The chef became suspicious and removed the leftover broth from the trash to investigate.
At Guildford Crown Court in southern England, Judge Christopher Critchlow said Cox would be sentenced March 4. She is free on bail.
Gold is one of Britain's richest women, whose chain of stores selling racy lingerie and sex toys has earned her a fortune estimated at 180 million pounds ($290 million). She built the business by shifting the focus of the company, founded by her father, from men to women.