Last year officers were allegedly fooled by a 29-year-old with a fake ID who passed by Carriage Gates, where PC Keith Palmer was stabbed by Westminster terrorist Khalid Masood in during his vile attack on the heart of British democracy.
The 29-year-old is believed to have then spent 12 hours wandering around the Houses of Parliament before being discovered the next morning drunk in the press gallery bar, which he had broken into.
The revelation of the alleged break-in comes as parliamentary figures called for security to be tightened at the entrance, which faces Parliament Square.
The 29-year-old is believed to have then spent 12 hours wandering around Parliament
One video showed the complex's gates were left open and apparently unmanned after Wednesday's terror attack.
Yards away, separate footage showed Prime Minister Theresa May being rushed from the building and into a waiting car with a clear line of sight between her and the terrorist Masood.
A parliamentary spokesman said Commons and Lords authorities were holding a series of "exceptional meetings" on security next week following the atrocity.
The intrusion comes as MPs call for security to be tightened at Westminster How the front pages of newspapers reported the London terror attack Thu, March 23, 2017
Media across Europe, to the USA, the Middle East have paid tribute to the victims
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One of the plans under consideration include adding further security outside Carriage Gates in a move which could close the road outside the Commons to all but parliamentary traffic.
A parliamentary spokesman said: "Following the creation of the Parliamentary Security Department in January 2016, security measures in and around the Parliamentary Estate have been reviewed, upgraded and enhanced.
"As is good practice following any significant incident, the Houses, in conjunction with the police and other bodies, are carrying out a review of security around this specific incident (Wednesday's terror attack).
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They were discovered the next morning drunk in the press gallery bar
Footage from Wednesday's attack showed Prime Minister Theresa May being rushed from the building
"A series of exceptional meetings are scheduled for next week including a joint meeting of the House of Commons and House of Lords Commissions where the details of the review are expected to be discussed.
"Security of members, staff and the visiting public is our highest priority. For Parliament to fulfil its democratic function, it is crucial that it remain open and accessible to the public."