The Duchess of Cambridge's armed protection squad surrounded a family on a day out when their car broke down near the Middleton family home.
Armed officers surrounded Adam De Marco's car after he stopped at the side of the road with a flat tyre, minutes from the home of the Duchess of Cambridge's parents Michael and Carole Middleton.
The Duchess, who is pregnant with her second child, has been staying at her parents' home in recent weeks while she recovers from severe morning sickness.
Adam, 40, his partner Claire Lennon, 39, and their daughter Mia, seven, were on their way to Bucklebury Farm Park in Berkshire when their BMW had a puncture. Within moments of the car stopping, armed officers swooped and surrounded the vehicle with their weapons drawn.
However, Adam told the Telegraph that the officers quickly put their guns away when they realised the family was not a threat - and then tried to help them repair their tyre.
Adam, director of a print company in Andover, Hampshire, said: "We got the flat tyre as we drove Mia to the farm park, where children can play on the swings and look at the pigs and other animals.
"I went to put on the spare, but then realised I was missing the special screw-nut you need with a BMW to remove the wheel - it's meant to stop thieves.
"I was in a quandary, wondering what to do...then all of a sudden we found ourselves surrounded by policemen who were carrying guns.
"I didn't twig at first that they were connected with the Royal Family, but one of them mentioned Prince William and the Middletons' house nearby and we put two and two together."
Adam said the officers were very helpful once they realised they were not a threat. He said: "They even drove off to visit a couple of garages in the area to see if anyone had a spare screw-nut so we could change the flat tyre.
"Unfortunately they came back empty-handed and said nobody had one, so we had to phone the RAC who arrived to changed the wheel.
"By that time it was too late to go to the park. Mia was a bit disappointed because she'd been looking forward to it, so we went back the next day.
"And while we were there, we saw Kate, her mum and Prince George at the farm shop, where you can buy fresh meat and other food.
"When they got into their Range Rover to drive off, we took some photos - and there was no sign of the protection squad."
Kate, 32, who is around 14 weeks pregnant, has been suffering with hyperemesis gravidarum and has been staying with Prince William and Prince George at the Middleton home Bucklebury.
She is now on the road to recovery and last week was able to make her first public appearance since announcing her pregnancy, riding in a carriage procession to Buckingham Palace as part of the state visit by the president of Singapore.
A spokesman for the Metropolitan Police - which provides armed protection officers for members of the Royal Family - said: "We do not discuss matters of operational security."