Gary Goldsmith, 52, admitted using a closed fist to hit Julie-Ann Goldsmith, who fell to the ground and was left semi-conscious outside their central London home in the early hours of October 13.
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.
He will be sentenced there on November 21.
Chief Magistrate Emma Arbuthnot indicated she would consider a community order over prison.
“But I am really looking at how to protect this lady from this man,” she added.
The court heard the couple began to argue while in a taxi after a charity event, then continued the argument on the street outside their home.
Prosecutor Kate Shilton said Goldsmith fell down after her husband threw what was described by their taxi driver, Daniel Shepherd, as a “left hook”.
“They both got out of the taxi and he (Mr Shepherd) describes Mrs Goldsmith slapping her husband to the face,” Shilton said.
“He then describes how Mr Goldsmith punches her hard in the face using a left hook.”
After falling to the floor, she remained flat-out with her eyes closed for around 15 seconds before waking up and staggering to her feet, the prosecutor added.
Her husband was “panicked and walking in and out of the house, trying to get her to go back in the house”, the court heard.
When Shepherd then challenged Goldsmith over his actions, he became aggressive towards the taxi driver, it was said.
Goldsmith’s wife subsequently asked the taxi driver to call the police.
At the police station, Goldsmith denied punching his wife, claiming he had pushed her hard with his left hand, but was apologetic for his actions, the court heard.