An investigation is under way following reports of an intruder spending the night at a military barracks close to Windsor Castle.
The British Army says the incident will be "thoroughly investigated".
The barracks are home to 1st Battalion the Coldstream Guards, whose ceremonial duties include standing guard outside Buckingham Palace and Windsor Castle.
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."