A woman was able to wander around the Houses of Parliament in the middle of the night after security guards fell asleep on the job, it is claimed.

In the latest in a series of embarrassing incidents, the middle-aged visitor evaded detection for several hours by hiding in the toilets after her official tour of the Palace of Westminster ended.

She then managed to walk around the corridors of power unchallenged in the early hours of Sunday morning before she was eventually spotted and police were called in to arrest her for trespass.

Now the Parliamentary authorities have launched an urgent investigation into how the shocking lapse happened – and it is likely to look into shock claims that guards meant to be patrolling the historic site were actually taking naps.

One source told the Daily Mail: 'This has caused a huge fuss as it's revealed that most of the night duty officers who should have been on patrol around the building were in fact asleep.'

It comes after an intruder climbed into the estate in broad daylight and was only stopped by a heating engineer once he had reached far inside the House of Lords.

The authorities have also been left embarrassed by a £10million front door that did not work and a 'hideous' fence, while MPs and peers were shocked when Director of Parliamentary Security Alison Giles abruptly quit earlier this year.

In the latest fiasco, a woman took part in an official tour of Parliament on Saturday afternoon but is then believed to have locked herself in a Westminster Hall toilet cubicle until long after visiting hours.

She did not emerge from her hiding place until about 2am on Sunday, it is claimed, then began wandering around Parliament.

However the authorities denied that she got as far as the Prime Minister's office in the corridor behind the Speaker's Chair.

The woman was found back in Westminster Hall, the oldest surviving part of the palace, by security officers who called for police help to arrest her.

A Metropolitan Police spokesman confirmed: 'In the early hours of Sunday, 7 June, 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.

'She was released on bail pending further enquiries.'

A Parliamentary spokesman said: 'A member of the public was arrested for trespassing on the Parliamentary Estate on Sunday 7 June. 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.'

The investigation is expected to focus on the Post and Patrol officers, the members of the Parliamentary Security Department who are meant to guard the inside of the estate.

During night shifts they are given areas to monitor and are meant to check toilets but it is claimed they failed to notice the woman as she was inside a cubicle that was marked closed for maintenance.

It is alleged that they then went off to rest rather than keeping guard throughout the night – and that the trespasser was then found by Perimeter Officers who usually focus on the outside of the estate and the main gates.

A source said: 'The management are likely to come down very hard on the officers who were on duty on Saturday night.

'They have now adopted a knee jerk reaction to this by making every officer who is on patrol to call in everywhere they've been to the control room, failure to do this is a disciplinary matter. Also, they are trying to sweep this under the carpet as it's embarrassing.'

Earlier this month it was revealed that 23 police officers have been served with misconduct notices over claims that those responsible for protecting the Royal Family at Windsor Castle fell asleep on duty.