The Ministry of Defence has launched an investigation after an intruder pretended to be a priest and stayed overnight with Royal Guardsman close to Windsor Castle.
Police were called to the site on Wednesday morning, where a man approached soldiers at the gate and told them he was a priest.
He then allegedly spent the night eating and drinking with senior officers before being offered a bed, without showing any ID.
A source told the Sun newspaper: "The guy turned up at the gate in the evening and said his name was Father Cruise and claimed to be a friend of the battalion's Padre Rev Matt Coles.
"He was invited in and offered something to eat in the Officer's Mess.
"Within a couple of hours, he was drinking with the officers in the bar and telling them stories of how he had served in Iraq.
"He was telling lots of tall stories and the lads were enjoying his banter and having a few drinks."
An Army spokesperson 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."
Thames Valley Police spokesman told the Sun: "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 Queen was at Sandringham for Easter at the time of the intrusion.
The barracks are home to The Coldstream Guards, well known for their red jackets and black bearskin hats and have a ceremonial role as protectors of Windsor Castle and Buckingham Palace.