It appears that security measures for Kensington Palace, the couple's new home, were posted on the website for London's Planning Department.
Buckingham has requested the information be taken down, but the department has refused.
"They cease to be security as soon as you let people have a look at them. I cannot see who would be interested in security arrangements in Kensington palace other than either nosy people or somebody wanting to break in," said Dickie Arbiter, Queen Elizabeth II's former press secretary.
The security breach comes at a time when the queen's own staff is being questioned about a dead body that was found on her rural Sandringham estate.