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The Houses of Parliament are among the most heavily-protected buildings in Britain.

All visitors must pass through heavy security and airport-style scanners before they are allowed in.

So how could an assailant carrying a weapon get inside to attack a policeman?

It appears the breach happened at Carriage Gates, the main access point for cars and other vehicles to the Parliamentary estate.

These huge iron gates are directly opposite Parliament Square and just round the corner from Big Ben, where hundreds of tourists mill about throughout the day.

(Image: PA) (Image: Twitter)

Many push their camera phones through the railings to take pictures of the famous buildings - but are nor allowed through unless they have a security pass.

However, these gates do open and close throughout the day to let selected vehicles in.

Indeed, less than three hours earlier Theresa May and her entourage had driven through the very same gates as she arrived for PMQs .

The gates are of course protected by standing policemen - but it is possible the attacker rushed past them and through the gates as they opened to let another car in or out.

It appears the man got 20 or 30 yards inside the gates and into New Palace Yard - inside the Parliamentary estate - before he was shot.

Questions will now inevitably be asked about whether security at Parliament's main entrances needs to be tightened.

(Image: AFP) (Image: REUTERS)

Expect a major review in the coming weeks. A crackdown looks certain to follow.

The police and Parliamentary authorities have always regarded the biggest risk to security in Westminster would be a Mumbai-style attack.

The 2008 terrorist incident saw militants storm the Taj Mahal hotel and randomly shoot guests and staff.

The atrocity led to a beefing up of security at Parliament. More armed police were placed at the main entrances and the gates for cars, which used to be left open, were now only raised on request.

The number of access points for passholders was restricted and all required the need for entrants to swipe their passes and enter a pin number before entering the estate.

Regular emergency drills were introduced, that included rehearsing lockdowns that would saw MPs and peers held in the chambers of the House of Commons and the House of Lords.