Royal protection officers in convoy with a car carrying Prince Harry on the motorway "put lives at risk" by driving recklessly at 100mph, it was alleged.
Company director Tim Williams, 33, has lodged a complaint with police, claiming that he was tailgated and forced to drive at dangerous speeds.
The businessman told the Mail on Sunday he was driving towards London on the M4 last Saturday with his wife, sister-in-law and eight-month-old baby.
He said: "They put lives at risk. It was about 8pm on Saturday, and we were driving from Marlborough, in Wiltshire, to London in the middle lane. I was about to pull out when I saw an Audi R8 in my mirrors. It came from nowhere. I let it pass. It was going a hell of a speed - at least 100mph.
"I then pulled into the outside lane. Immediately a Range Rover came right up behind us. It was very close. I thought they were going to nudge me. It was frightening. The middle lane was busy so I couldn't pull back in.
"I accelerated because the Range Rover was so close, ridiculously close. I thought perhaps the Range Rover was racing with the Audi. It really did scare us. It was intimidating."
Mr Williams told the newspaper he realised it was an unmarked police car when the blue flashing lights came on in the radiator grille. He said he had no option but to accelerate to speeds he was uncomfortable with - well over 70mph.
When the Range Rover eventually overtook him at more than 100mph the angry passenger shook his fist at him, Mr Williams said. But according to the businessman, 20 minutes later he caught up with both cars in traffic in west London and was shocked to see the Prince in the passenger seat of the Audi, putting on sunglasses and shielding his face.
A Thames Valley Police spokeswoman said: "We can confirm that a complaint was received by Thames Valley Police on Wednesday May 14. The complainant was advised to contact the Metropolitan Police Service.
"Thames Valley Police continues to liaise with the Met in relation to the complaint. It's being looked into."
