An urgent inquiry has been launched after a middle-aged woman managed to roam freely through the Houses of Parliament during the early hours of Sunday morning after security guards "fell asleep".
The visitor, said to be in her 50s, concealed herself inside a toilet cubicle in Westminster Hall following an official tour on Saturday afternoon.
She remained hidden until 2am, before emerging to walk through the corridors of the Palace of Westminster - without being challenged.
Security staff eventually discovered her back in Westminster Hall, the palace's oldest surviving section, and contacted police.
Officers arrested the woman on suspicion of trespassing, where she has since been released on bail while investigations continue.
It is said the woman avoided detection by locking herself inside a cubicle adorned with an out-of-order maintenance sign.
According to the Daily Mail, most of the night duty officers who should have been on patrol around the building were in fact asleep.
Guards responsible for monitoring the interior are reportedly assigned specific zones and required to inspect toilet facilities during their shifts, which wasn't conducted in this instance.
Security staff eventually discovered the visitor back in Westminster Hall and contacted police
| GETTYThe trespasser was ultimately discovered not by internal patrol staff, but by perimeter officers, whose duties involve defending the estate's exterior boundaries and entrance gates.
A Met Police spokesman confirmed: "In the early hours of Sunday, June 7, a woman in her 50s was arrested on suspicion of trespassing - breaching a section of the Serious Organised Crime and Police Act 2005 - after remaining in the Palace of Westminster for several hours following the conclusion of an organised tour."
A Parliamentary spokesman echoed: "A member of the public was arrested for trespassing on the Parliamentary Estate on Sunday June 7. The incident is now being dealt with by the Metropolitan Police.
"The safety and security of all those who work and visit in Parliament is our top priority. Whilst we cannot comment on our security processes or measures, a review is underway following the incident".
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The inquiry is expected to examine the conduct of post and patrol officers of the Parliamentary Security Department, tasked with guarding the building's interior.
Authorities denied claims that the woman reached the Prime Minister's office located in the corridor behind the Speaker's Chair.
The incident serves as the latest in a string of security embarrassments for Parliament, where a previous breach saw one intruder scale the estate perimeter in broad daylight, then moving through the House of Lords, before being intercepted by a heating engineer.
The parliamentary estate has also faced criticism over a malfunctioning entrance which cost £10 million, and a fence described as "hideous".
MPs and peers were further rocked when Director of Parliamentary Security Alison Giles departed suddenly earlier this year.
The security blunder comes following revelations earlier this month that 23 police officers have received misconduct notices over allegations of sleeping while guarding the Royal Family at Windsor Castle.