Kate Middleton's uncle, Gary Goldsmith, admitted to physically assaulting his wife, Julie-Ann Goldsmith, on the 14th of November.
Goldsmith - the younger brother of Carole Middleton, the duchess' mother, and a guest at Kate's 2011 wedding to Prince William - appeared at Westminster Magistrates' Court to admit one count of assault by beating.
Gary Goldsmith, 52, struck Julie-Ann Goldsmith, 47, with a "left hook" following a night at a private members' club in central London.
Prosecutors are seeking a restraining order to ban Mr Goldsmith from contacting his wife, suggesting their marriage is over.
Prosecutor Kate Shilton said Mrs Goldsmith fell down after her husband threw what was described by their taxi driver, Daniel Shepherd, as a "left hook'".
During the taxi ride, Mr. Goldsmith and Julie-Ann began shouting at one another after they had been drinking together the entire evening. He then describes how Mr Goldsmith punches her hard in the face using a left hook'.
After having lost consciousness for long seconds, she was back at it and had then asked the taxi driver to call the police. He pleaded guilty to assault by beating.
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The incident occurred in the month of October last, as he returned from a charity event which he attended with his wife, Julie-Ann.
Ms Shilton said Goldsmith appeared to be "panicked" and when the taxi driver challenged him over his actions, he became aggressive.
The couple had been at a charity auction at Home House in Portman Square, Marylebone before they clashed.
When at the police station Goldsmith said he had pushed his wife hard but denied hitting her.
Goldsmith will be sentenced at the same court next Tuesday.
Chief magistrate Emma Arbuthnot told a probation officer who will assess him: "I'm really looking at how to protect this lady from this man in the future, so it doesn't happen again". Speaking after meeting Kate, Billy said: "She was saying William was sort of similar and I just wanted to let her know how Family Action have been there for me and I think they need more support". Their mother worked in a shop and also did accounts, while their father was a painter and decorator.