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Kate Middleton's uncle has been arrested on suspicion of assaulting his wife in the street.
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A cabbie said he saw businessman Gary Goldsmith, 52, hit Julie-Ann after a row.
Taxi driver Daniel Shepherd said Julie-Ann, 47, was knocked out in the incident in the early-hours of Friday outside the couple's central London home.
Wife Julie-Ann was reportedly left unconscious after smashing her head on the pavement, the cabbie said tonight.
Police and an ambulance arrived and she was taken to hospital.
Goldsmith - the younger brother of Kate’s mum Carole Middleton - was arrested at the scene on suspicion of domestic assault.
Police confirmed he was later charged and is due in court on October 31.
Mr Shepherd, 28, said he witnessed the drama unfold after driving the couple to their flat in posh Kensington, central London.
He said the pair began arguing in his taxi and continued as he dropped them off at their home shortly before 1.30am.
Daniel told The Sun newspaper: "He was screaming and swearing at her and she slapped him round the face and his glasses fell off.
"It kicked off as they tried to unlock door.
"He then turned and hit her in the face with a left hook.
"She fell and cracked her head on the pavement."
Goldsmith met the royal family when Kate married Prince William in 2011.
In the past he has spoken fondly of his nieces. Describing his pride at seeing Kate marry The Duke of Cambridge, he said: "She is this confident, clever, caring girl who has taken her responsibilities in marriage and job incredible seriously.
"I think everyone thinks she's sincere, in love and doing her job to the best of her ability."
Goldsmith was brought up with his sister in Southall, west London.
Their mother worked in a shop and also did accounts, while their father was a painter and decorator.
Mr Shepherd said after Goldsmith appeared to punch his wife, he jumped out of his cab and yelled "You can't do that!" before dialling 999.
Goldsmith allegedly responded by using his own phone to take pictures of Mr Shepherd's cab and said "I'm going to report you."
Julie-Ann was treated by paramedics and appeared to have a black eye, Mr Shepherd claimed.
After police arrived, officers led Goldsmith from his home in handcuffs to a police station.
"He has been bailed to appear at Westminster Magistrates’ Court on Tuesday, 31 October."
Julie-Ann took her dogs for a walk in Regent’s Park on Friday morning, but declined to comment when approached by the Sun.
In an interview four years ago, Goldsmith said: "People like to think I'm a bad boy. I’m not really."