Scotland Yard has apologised for the "wholly unacceptable" breach of security at Windsor Castle on Saturday evening.
The intruder got in by climbing a tree and jumping on to a wall
Here is the full statement released by Metropolitan Police Assistant Commissioner David Veness:
"The events at Windsor Castle are wholly unacceptable and a matter of great regret.
"The Commissioner has tendered apologies to the Royal Family for the appalling breach of security at Windsor Castle on Saturday evening.
"It now appears that the intruder gained access to the castle precincts by scaling an embankment, climbing a tree, jumping onto a wall and then reaching a terrace.
"He was challenged by a contractor and escorted to a police point.
"He gave what appeared to be a credible story and was allowed into the area of the castle where the party was taking place.
"This seems to have been an operational failure, which should not have happened whatever the circumstances.
"An initial report on events will be delivered to the Home Secretary today (Monday).
"We have already established an independent inquiry led by Commander Frank Armstrong of the City of London Police to rigorously review all aspects of the security operation and its implementation for this event.
"He will make recommendations. Commander Frank Armstrong is an expert on protection matters.
"You will understand that I cannot comment further on matters that are the subject of criminal and discipline investigations."