Kate Middleton’s uncle was arrested on suspicion of thumping his wife in the street.
A taxi driver said he saw businessman Gary Goldsmith, 52, hit Julie-Ann after she accused him of doing cocaine.
Cabbie Daniel Shepherd said Julie-Ann, 47, was knocked out in the early-hours bust-up outside the couple’s central London home.
Multi-millionaire businessman Gary Goldsmith — younger brother of Kate’s mum Carole — was arrested on suspicion of domestic assault.
His wife, who was allegedly hit in the eye, was taken to hospital by ambulance and treated for her injuries before being discharged.
Cabbie Daniel Shepherd, 28, said he witnessed the drama after driving the couple to their flat in central London following a night at the exclusive Home private members’ club in Portman Square, Marylebone. He said the pair began arguing in his taxi after Julie-Ann accused Goldsmith of snorting cocaine.
The row continued as he dropped them off at their Georgian home shortly before 1.30am. Daniel said: “He was screaming and swearing at her and she slapped him round the face and his glasses fell off.
“He then turned and hit her in the face with a left hook. She fell and cracked her head on the pavement.”
Goldsmith’s arrest will shock the Royal Family, whom he met when Kate wed Prince William in 2011.
Daniel told how he picked up IT recruitment boss Goldsmith and 47- year-old Julie-Ann at around 1.15am after they hailed a cab for their £6, five-minute ride home.
He said: “They seemed to have had a good drink and she was upset with him and saying, ‘I haven’t seen my husband for a week and you go and leave me to do cocaine with your business buddies’.
“She said, ‘Do you think I’m stupid’? He started yelling at her and saying, ‘You’re a dirty whore’.
“He was being rude to me as well and boasting of spending £100 a day on cabs. I pulled up at their home, she got out and he paid the fare.
“He then got out and started taking over from her to unlock the front door. That’s when it kicked off.