MASKED raiders struck on the Windsor Castle estate as the Prince and Princess of Wales and their children slept in their home nearby.
Two men scaled a 6ft fence at night to steal farm vehicles.
A source said: “They must have been watching Windsor Castle for a while.”
The fleeing thieves used a stolen truck to smash their way through a security gate at the royal estate.
Two masked men drove off in a pick-up and on a quad bike that were stored in a barn.
King Charles and Queen Camilla were not in residence.
But the Prince and Princess of Wales, both 42, were believed to be at Adelaide Cottage with George, 11, Charlotte, nine, and six-year-old Louis.
It is just five minutes away and the family are regularly seen using the wrecked gate — the nearest exit to their home.
News of the break-in comes after The Sun on Sunday revealed armed police were removed from Windsor Castle’s two main public entrances, though gun cops are still thought to patrol the grounds.
The raiders struck at Shaw Farm, a working farm inside the Castle security zone.
A source said: “They would have to have known the vehicles were stored there before they broke in and known when was the best time to get and escape without being caught.
“So they must have been watching Windsor Castle for a while.
“There are alarms at Windsor Castle but the first time anyone knew there was a break-in was when they crashed through the security barrier at Shaw Farm Gate exit.
“The barrier has needed to be replaced as it was so badly damaged.
“It was a Sunday night on a school week so the young princess and princess would have been tucked in bed at Adelaide Cottage, just around the corner but still in the grounds.”
William and Kate moved to the Cottage in the summer of 2022.
During the late Queen Elizabeth’s funeral, the procession to Windsor Castle began from the Shaw Farm gate.
The decision to remove armed police from it and the other main entrance for the public and tradespeople was said to be down to a shortage of qualified officers.
It was also to soothe tourists nervous about gun-toting cops.
Armed police now only appear at the front of the castle at the Changing of The Guard ceremonies.
Last night a spokesman for Thames Valley Police, confirmed: “At around 11.45pm on Sunday October 13, we received a report of burglary at a property on Crown Estate land near to the A308 in Windsor.
“Offenders entered a farm building and made off with a black Isuzu pick-up and a red quad bike. They then made off towards the Old Windsor/Datchet area.
“No arrests have been made at this stage and an investigation is ongoing.”
The King was in Scotland during the raid and it is believed Camilla was at a spa in India ahead of the couple’s tour of Australia and Samoa.
A spokesman for the Prince and Princess of Wales declined to comment.
Previous Windsor Castle security scares include one on Christmas Day 2021, when an intruder armed with a crossbow climbed a fence and claimed he wanted to kill the Queen.
Jaswant Singh Chail, 23, was jailed for nine years for treason.
Read more on the Scottish Sun
In February, an unarmed man was found in the gardens outside the King’s Windsor Castle apartments.
The man was arrested on suspicion of trespass and detained under the Mental Health Act.