A man has been arrested after scaling a 12-foot fence and gaining entry to the royal state rooms of Buckingham Palace.
He was found "in an area currently open to the public during the day" at about 10.30pm BST on Monday, a Metropolitan Police spokesman said, and was arrested on suspicion of burglary, trespass and causing criminal damage.
Police and security rushed to the scene after motion sensors were set off. The state rooms contain many priceless works of art.
A suspected accomplice was arrested outside on suspicion of conspiracy to commit burglary.
No members of the royal family were in the palace at the time of the incident and police said that "a review of the specific circumstances of this incident" was being carried out, which would include a review of palace security.
Buckingham Palace has not released a statement.
The Queen has been spending her summer break at Balmoral, and is not expected to return to the palace until October.
On the accession of Queen Victoria in 1837, Buckingham Palace became the centre of the British monarchy, and is now one of the Queen's three official residencies as well as being the royal family's administrative centre.
Works by Van Dyck and Canaletto hang in the 19 sumptuous state rooms, where the monarch entertains visiting dignitaries. Over the summer, they are open to public tours.
A source close to the investigation quoted by the Sun said: "This breach of security is being treated with the utmost seriousness.
"There are many red faces at the fact that a man jumped over the gates and gained access unchallenged."
The break-in is one of the most serious since unemployed father of four Michael Fagan broke in to the palace in 1982 and walked into the Queen's bedchamber while she was asleep.
He spent about 10 minutes chatting to the Queen before she managed to alert a footman after Fagan asked for a cigarette and she left the room.
In 2003, 'comedy terrorist' Aaron Barschak broke in and gatecrashed Prince William's 21st birthday party at Windsor Castle.
In 1997, an escaped psychiatric patient managed to get into the grounds of Buckingham Palace, and in 1993 a group of anti-nuclear protesters were arrested after they managed to scale the walls of the palace.
Last year a Daily Mirror journalist gained entry to the palace by working undercover as a palace footman.