For a tense half a minute the Queen’s 53-year-old son was in danger of being shot dead by officers paid to protect him and his family.
Last night a internal inquiry was being carried out into the extraordinary incident, which took place last Wednesday.
Royal protection officers were said be “jittery” after a genuine intruder had been found in the Palace’s state rooms two days earlier. A suspected accomplice was also arrested in a street outside the Palace.
Speaking exclusively to the Sunday Express, a royal insider said: “Everyone is talking about these two incidents and can’t quite believe what has happened.
“The fact that this guy was able to wander off unchallenged and not be spotted by anyone is bad enough. But then to mistake the Duke for another intruder is almost incomprehensible.
“There is a high turnover of the police on duty at the Palace, but you’d think anyone would know what Prince Andrew looks like.
“It’s fair to say that to describe the Duke as unhappy and the two officers as highly embarrassed is the biggest understatement of the century.”