An Afghan immigrant smuggled himself into Britain via Princes William and Harry's former military school.
The 20-year-old man is believed to have slipped onto a coach packed with officers en route to the Sandhurst Royal Military Academy when it stopped in Calais, France.
The man is thought to have hidden in the stairwell near the toilet and was only discovered once the vehicle was inside the famous Sandhurst complex in Camberley, Surrey.
He had already managed to get past border control in Dover and security at the college.
The man was only discovered when a corporal and a lance corporal saw him hiding as they got off the bus.
Chief of the General Staff General Sir Richard Dannatt is said to be "incandescent" over the security breach, while sources have branded the incident "extremely serious".
General Sir Anthony Walker, former Deputy Chief of the Defence Staff, said: "All vehicles entering the academy should be searched. There are enough armed soldiers and uniformed MoD personnel on the gate to do this.
"Fortunately there was no ghastly outcome of the security breach. But one day there could be."
The coach was reportedly waved through the gates with minimal checks.
Police were called to the academy and the man was taken to nearby Maidenhead police station.
It is expected he will be sent to a detention centre and then deported.
A military source told Britain's The Sun newspaper: "He could have smuggled a bomb inside Sandhurst - or could even have been a suicide bomber, which would have been catastrophic."
A spokesperson for the Ministry of Defence blamed the coach contractor for the incident.
They said: "We can confirm that an individual was discovered onboard a coach within the Royal Military Academy Sandhurst having stowed onboard during the coach's transit from Germany.
"The coach was provided and driven by a civilian contractor who has responsibility for the security of their vehicle. Security officers in all Army establishments have been informed and reminded of security procedures to reduce the risk of a further incident of this nature."
They were unable to confirm the man's nationality.
Shadow immigration Minister Damian Green commented: "Even after all the security disasters of recent years, it still beggars belief to find that an illegal immigrant can not only smuggle himself into this country, but into Sandhurst on an Army bus."
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