Footage of the aftermath of the Westminster attack shows Parliament's gates unmanned as a motorcyclist makes his way in.
The man, who has not been identified but may be a courier, rode through the open Carriage Gates where attacker Khalid Masood had struck minutes earlier.
The video captured by a journalist from The Times, who witnessed the events from an office window on Wednesday, raises fresh concerns about security at the Palace of Westminster.
Wounded PC Keith Palmer can be seen on the ground in New Palace Yard, metres away from Masood, who was shot dead by a close protection officer for the Defence Secretary.
Theresa May, who was in the voting lobby at the time, was around 100 yards away from the scene, being bundled into a car by security and taken away.
Carriage Gates have been repeatedly identified as a weak spot in Parliamentary security, often open for ministerial cars and parliamentary staff to pass through.
It is understood that armed police used to be stationed at the gates until around two years ago, when a decision was taken to replace them with a mobile patrol.
Police sources told The Times the change in arrangement was the result of MPs complaining that their presence gave the wrong image of Parliament.
One source was quoted as saying: "This was a fixed point but that changed following political pressure.
"They wanted a traditional British bobby on the gate; they wanted that image - but this is a sensitive security area. If there had been two armed officers on the gates then they would have taken the shot. Thank God there was only one nutter in that car."
Concerns were raised by MPs and staff that firearms officers were "off-putting" and created a "bad atmosphere", according to evidence presented to the Commons Administration Committee in 2014.
Commons authorities said they liaise with the police and security services about the arrangements, and would not confirm if the change in security was related to complaints.
A security assessment presented to MPs last month identified Carriage Gates as a weak point, and security there was due to be reviewed this year.
Nigel Evans MP, the former deputy Speaker of the Commons, told Sky News: "I was horrified to see the footage, the bodies on the floor. And the vehicle is the weapon of choice now, which means we need to look again at basic security.
"I think there's about eight or nine points of entry for members of Parliament and they all need to be looked at afresh.
"We have unarmed officers at the front, they are the first point of contact with the public and people like to take selfies with them and it's almost a tradition.
"But there have to be armed guards where there are areas of weakness. A terrorist isn't going to use the public entrance with security scanners.
"There are lots of armed police around and they do a fantastic job, but if unarmed bobbies remain in place, within inches there need to be fully trained and armed police - not just to protect MPs but to protect democracy and the thousands of people who visit the estate including children.
"There were schoolchildren in the building when this happened."