Burglars raided the Windsor Castle estate while the Prince and Princess of Wales and their children reportedly slept nearby.
Two men are said to have climbed a 6ft fence at night and stolen farm vehicles during the raid last month, The Sun reported.
They used a stolen truck to break through a gate said to be the nearest exit for the Prince, Princess and their children before targeting Shaw Farm, a working farm inside the estate.
The pair, who remain at large, stole a black Isuzu pick-up and a red quad bike before making off, police said.
On Sunday night, a Thames Valley Police spokesperson told the newspaper: “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.”
Kensington Palace did not comment to the newspaper on Sunday night.
The Prince and Princess are reported to have been at Adelaide Cottage, which they bought in 2022, at the time.
The King is said to have been in Scotland, while the Queen is said to have been in India before the royal couple’s tour of Australia and Samoa.
A source told The Sun: “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 princes and princess would have been tucked in bed at Adelaide Cottage, just around the corner but still in the grounds.”
It comes after Jaswant Singh Chail, 23, climbed a fence on castle grounds and said he wanted to kill the late Queen on Christmas Day 2021.
Last year, he was jailed for nine years for treason, becoming the first person to be convicted of the offence since 1981.
