British police said on Saturday they had arrested a man who scaled a fence to get into Buckingham Palace, the official London residence of Queen Elizabeth II.
No members of the royal family were in the palace at the time of the incident on Monday night, said a spokesman for London's Metropolitan Police.
The man was found "in an area currently open to the public during the day" and arrested for burglary, trespass and criminal damage, the spokesman said.
A second man was arrested outside the palace in central London for conspiracy to commit burglary.
"A review of the specific circumstances of this incident is being carried out," the police spokesman said.
Both men were bailed to return to a London police station at a later date.
It was not immediately clear why the incident had only come to light five days after it happened.
Buckingham Palace declined to comment.
It is one of the most serious security breaches at the palace since 1982 when unemployed Michael Fagan got inside the queen's private chambers while she was in bed.
Fagan spent 10 minutes talking to the queen after climbing over the palace walls and up a drainpipe, and she raised the alarm when Fagan asked for a cigarette.
Queen Elizabeth, 87, is currently spending her summer break at Balmoral Castle in Scotland where she is expected to make a traditional appearance at the Highland Games on Saturday.