A 22-year-old man has been arrested after climbing over the front gates of Buckingham Palace while the Queen was in residence.
The man had successfully scaled the gates before being caught at around 2am on Wednesday morning, police confirmed, and has been arrested on suspicion of trespass.
The incident is not being treated as terror-related, with the man remaining in custody at a central London police station.
It will raise serious security concerns for the palace, and comes almost exactly 37 years after Michael Fagan famously broke into the Queen's bedroom.
The Sun newspaper reported that this time, the man had spent around four minutes inside palace walls and had banged on the doors in an attempt to break in before being caught by officers from the Met's Royalty and Specialist Protection Command.
A spokesman for the Met Police said: "Officers have arrested a man on suspicion of trespass, contrary to section 128 of the Serious Organised Crime and Police Act 2005.
"The man was not found in possession of any offensive weapons and the incident is not being treated as terrorist-related."
The Queen, who was asleep in the palace during the incident, will have been informed, but continued with her duties on Wednesday to present the Queen's Medal for Music.
She received jazz musician Gary Crosby and Judith Weir, Master of The Queen's Music, at Buckingham Palace.
On July 9, 1982, an unemployed decorator named Michael Fagan scaled the palace walls and wandered into the Queen's bedroom in what has now become a notorious account of Her Majesty keeping a cool head.
Then, she is said to have engaged him in conversation as he sat on the end of her bed, before a footman and police came to the rescue.
In recent years, police have had to deal with security scares including several arrests of members of the public outside palace gates with knives.
In 2013, a man was arrested for burglary, trespass and criminal damage, caught in one of the state rooms after scaling a 12ft fence.
Last year, a 38-year-old visitor to the palace arrested on suspicion of having a firearm faced no action after police were satisfied he did not realise his keyring contained a taser.