The Army has launched an investigation after an intruder reportedly spent the night at a barracks close to Windsor Castle, alongside Royal guardsmen.
The Sun newspaper reported that police were alerted to reports of the intruder at Victoria Barracks, home to the Coldstream Guards, in Windsor at 09:20 BST last Wednesday.
The man approached soldiers pretending to be a priest, Talk TV reported.
The Army said the matter would be thoroughly investigated.
The barracks is close to Windsor Castle, where the Queen has been living, but at the time of the incident the monarch was in Sandringham, in Norfolk, for Easter.
The man is reported to have spent the night eating and drinking with senior officers, having claimed to have been a friend of the battalion's padre, according to Talk TV.
The Army spokesman said: "The Army takes this breach of security extremely seriously and it will be thoroughly investigated as a matter of priority.
"This incident is now part of an ongoing investigation and it would therefore be inappropriate to comment further at this time."
A Thames Valley Police spokesman said: "We received a report of an intruder at Victoria Barracks in Sheet Street, Windsor, at 9.20am on Wednesday.
"Officers attended and removed the intruder from the barracks. No further action was required."
The Coldstream Guards, who wear red jackets and bearskin hats, have a ceremonial role as the protectors of Windsor Castle and Buckingham Palace.