Masked men scaled a 6ft fence to sneak onto the Windsor Castle estate as the Prince and Princess of Wales slept in their cottage just a stone's throw away.
The two men reportedly used a stolen truck to smash through a security gate before stealing farm vehicles and driving off in a pick-up and on a quad bike that they took from a barn at Shaw Farm.
Just five minutes away, William and Kate and their children George, 11, Charlotte, nine and Louis, six, are thought to have been peacefully asleep.
The now-wrecked gate is the nearest exit to Adelaide Cottage and the family are often seen using it to come and go.
It comes after armed officers were removed from the public entrances to Windsor Castle.
But the officers are still thought to be present, patrolling the grounds of the estate.
A source said the masked thieves must have been 'watching Windsor Castle for some time'.
They told The Sun: '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.
'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.'
The King was in Scotland and Camilla was said to be in India at the time of the raid.
A spokesman for Thames Valley Police said: '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 Wales family moved from Kensington Palace to Adelaide Cottage, located within Windsor Great Park, in the summer of 2022.
Their preference was for a residence that required no renovation or added security arrangements. In contrast, Prince Harry and Meghan Markle spent £2.6m fixing up nearby Frogmore Cottage.
Additionally, four bedrooms were deemed sufficient since there are no live-in staff members. Instead, aides commute to the property as and when their services are needed.
Nestled at the heart of the Crown Estate's private 655-acre royal park, Adelaide Cottage was built in 1831 as a retreat for William IV's wife Queen Adelaide of Saxe-Meiningen.
It was also known to be a favourite home of Queen Victoria, as she frequently enjoyed taking her breakfast there.
However, the most famous former resident has to be the late Princess Margaret's ex-lover, Group Captain Peter Townsend, whose affair with the Queen's sister caused a national scandal.
The cottage underwent major renovations in 2015, which meant that Prince William and Kate did not need to shell out millions in remodelling the house.
Adelaide Cottage still boasts original features, including a marble Graeco-Egyptian fireplace and a principal bedroom, with a coved ceiling featuring gilded dolphins and rope ornament reused from the Royal yacht Royal George.
The procession to Windsor Castle during the late Queen's funeral began from the Shaw Farm gate, which has since been replaced after being mowed down during the raid.
Other Windsor Castle security scares include one which occurred on Christmas Day 2021, when a crossbow intruder climbed a fence and claimed he wanted to kill the Queen.
Jaswant Singh Chail, 23, was jailed for nine years for treason.
MailOnline contacted Kensington Palace for comment.
