An intruder pretending to be a priest reportedly spent the night at barracks with Royal Guardsman close to Windsor Castle.
The Sun reported police were alerted after the unknown man arrived at Victoria Barracks in Sheet Street, Windsor, at 9.20am on Wednesday while the Queen was at Sandringham for Easter.
The barracks are home to The Coldstream Guards, recognised by their red jackets and black bearskin hats and have a ceremonial role as protectors of Windsor Castle and Buckingham Palace.
The man allegedly spent the night eating and drinking with senior officers before being offered a bed for the night, despite showing no identification or credentials.
A source told the newspaper: “This is just an extraordinary breach of security.
“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.
‘Enjoying the banter’
“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 an investigation had been launched.
A statement added: “The Army takes this breach of security extremely seriously and it will be thoroughly investigated as a matter of priority.”
Thames Valley Police spokesman told the Sun they had been alerted on Wednesday.
“Officers attended and removed the intruder from the barracks. No further action was required.”