A 22-year-old man has been arrested after climbing over a security fence at Buckingham Palace.
The man, who had been drinking, was spotted on CCTV and detained at around 4.15am on Tuesday, Scotland Yard said.
He was stopped "within a security perimeter" and did not manage to get into the palace.
The Queen is currently at Balmoral in Scotland.
Scotland Yard said the man was unarmed and is in custody at a police station in central London.
The breach is not believed to be related to terrorism.
It is understood that no members of the Royal Family were in residence at the time.
Buckingham Palace declined to comment on the matter.
In May, there was another security breach when convicted killer Denis Hennessy scaled a perimeter wall and spent around 10 minutes ''admiring'' the Palace gardens before being apprehended by armed police and repeatedly asking ''Is Ma'am in?''
Hennessy, of Barnhill Road, Wembley, north-west London, was jailed for four months at Westminster Magistrates' Court for criminal damage and trespassing on royal grounds.
It emerged that he had been jailed for life in 1993 for beating homeless man Kevin Cusack to death the previous year.
He was held about 50 metres from the Palace after triggering a ''full-scale alert'' involving a sweep of the grounds with dogs and a helicopter.
Commander Adrian Usher, head of the Metropolitan Police's specialist protection, said Hennessy had been arrested "without incident" and that security measures had "worked effectively".