The Daily Mirror newspaper reporter Ryan Parry spent two months working undercover in Buckingham Palace, while British officials made plans for the president's state visit.
Parry used fake references to get the job, and was in the palace last night when Bush arrived.
The reporter watched as the president and his wife, Laura Bush, were received by the Queen and the Duke of Edinburgh in the palace gardens at 2000 GMT.
The journalist, along with other members of Buckingham Palace staff, was given an itinerary of the president's movements.
The newspaper says that in the wrong hands such information could prove disastrous.
The Daily Mirror says that Parry's details were never properly checked out by the palace.
An internet search would have shown the reporter's name and photograph relating to some of his previous Daily Mirror investigations.
The security blunder will cause huge embarrassment to Buckingham Palace and royal security staff who allowed an intruder to gate-crash Prince William's 21st birthday party at Windsor Castle two months ago.
The newspaper is calling for a top-level investigation in to the latest security blunder.