The footage, captured by The Times, shows the aftermath of the assault on New Palace Yard which left Pc Keith Palmer mortally wounded.
As armed officers swarm the cobbled forecourt having shot dead terrorist Khalid Masood, the imposing iron gate which allows vehicles to enter can be seen wide open.
No police officers are visible guarding the entry point, known as Carriage Gates, fuelling concern the attack might have been worse had Masood been followed by accomplices.
Pedestrians are shown walking past and at one stage a courier on a moped appears to enter unchallenged.
Yards away, separate footage showed Theresa May being rushed from the building and into a waiting car.
Although the gate was open for a matter of minutes, critics will use the brief security lapse to rebuke claims on Friday by Scotland Yard’s anti-terror chief that current arrangements were “proportionate”.
Mark Rowley told reporters that procedures for guarding Parliament had been designed so they were not “overly intrusive”.
“Our current arrangements have been developed with Parliament over many years and are designed to provide access to the seat of our government balanced with security that is proportionate but not overly intrusive,” he said.
Parliament’s main entrance has two sets of large metal gates allowing vehicles to go in and out of the estate and they have traditionally been left open during the day.
A pair of smaller, makeshift gates was introduced more recently with two police officers at each to check passes and allow cyclists, cars and delivery drivers to come and go.
Just inside the entrance gate, armed police are usually present and an unarmed officer sits in a booth by the exit.
Electronic ramps are depressed and barriers lifted further into the courtyard after passes are checked using handheld machines which flash up with a picture of the passholder.
More police, some armed, are usually present after the final checkpoint.
Former Commons deputy speaker Nigel Evans said security must be boosted after the attack, and described the outrage as “one of those things that by experience you learn” from.
He revealed that “lots” of MPs locked in the Commons chamber during the attack were discussing how to boost security in certain areas, but said far more checks are carried out at Carriage Gates than when he was elected in 1992 and a “bobby” would just “wave you through”.
The Tory MP told BBC Radio 4’s Today programme: “I’ve got no doubts whatsoever that, under the Speaker (John Bercow), that he is regularly in touch with the Metropolitan Police, and that they will be talking about now, new security measures that will be there.”
Mr Evans added: “I’ve got no doubts whatsoever that there will be enhanced features of security, it’s happening on a regular basis, but following this tragedy security has got to be upped at the same time as still having a welcoming hand to members of the public to come and see how democracy works.”