Questions continue to be asked about whether Wednesday's fatal terror attack in Parliament could have been prevented if more officers had been armed.
Attention is focusing on how Khalid Masood was able to enter the Carriage Gates entrance of Parliament before he was confronted and subsequently shot.
One security expert said the breach may have been a product of complacency.
Ministers insist the police did their job and Parliament can't be sealed off - and a security review is under way.
Read what we know about the Westminster attack
A Parliamentary spokesman said security was always under "constant review" but the building had to remain "open and accessible" for it to function.
The review follows the fatal stabbing of PC Keith Palmer in the grounds of the Palace of Westminster.
Ministers have declined to comment on unconfirmed reports that his alleged assailant was shot dead by a member of Defence Secretary Sir Michael Fallon's close protection team, who happened to be at the scene, rather than a uniformed officer stationed at the gate.
Some MPs regard the carriage gates entrance, which is used by government cars to drop off and pick up ministers, as a weak point in the security perimeter.
It is manned by two police officers but they are not always armed and the gate is often open, unlike other entrances to the Palace of Westminster.
The Times reported that a 2005 review had found the entrance to be a weak point, primarily due to the number of delivery vans using it.
The paper quoted parliamentary sources as saying any changes at the time had been regarded as "too cumbersome".