A statement from the Metropolitan police said: “Shortly before 13.45 hours on Friday, police at Buckingham Palace were alerted to a man climbing over the south gate.
“As he dropped to the ground he was met by officers and arrested for trespassing on a designated site. He was not found in possession of any offensive weapons.
“The 21-year-old man was taken into custody and will undergo a mental health assessment.”
It is understood that no members of the Royal Family were in residence at the time of the security breach. The Queen was at an investiture ceremony at Windsor Castle earlier in the day.
It is the third time this year that a person has tried to enter the palace grounds.
In August, a 22-year-old man was spotted on CCTV after climbing over a security fence but was stopped “within a security perimeter”. A few months earlier, a 41-year-old man spent seven minutes in the grounds before being arrested.
The Queen’s London home has long been a target for intrusions, with security breaches at Buckingham Palace ranging from a naked paraglider landing on the roof to an undercover journalist posing as a footman.
The most notorious was when unemployed decorator Michael Fagan made his way over the walls and up a drainpipe, and wandered into the Queen’s bedroom in 1982. She woke to find Fagan sitting on her bed.