By Nick Assinder
BBC News Online political correspondent
It is the Serjeant-at-Arms Sir Michael Cummins - and not the Metropolitan police - who is ultimately responsible for security in the Commons chamber.
Protesters were within inches of frontbench
The role was created in the middle ages and then, as now, reports directly to the Commons Speaker.
Sir Michael and his staff of officials have a uniform of knee breeches and swords, which appear to many as quaint throwbacks to a previous age.
They certainly appear out of place and probably inadequate when compared to the machine gun wielding police officers who patrol the precincts and search visitors to the Palace.
Reports they were about to be replaced by a new security force have so far proved unfounded.
But Wednesday's security breach - the most serious in living memory - is certain to spark calls for this ancient arrangement to be brought to an end, in favour of something more modern.
A group of hunt protesters stormed into the Commons chamber shortly after 1620BST to face ministers eyeball to eyeball and scream abuse at them.
The security staff - wearing their traditional black tails and at least one sporting a sword - did stop three protesters and quickly wrestled the five who broke into the chamber to the ground and out of the chamber as MPs looked on in shocked disbelief.
But the five who made it to heart of British democracy were there long enough to have caused deaths or serious injury had they been determined to do so.
This was a far more serious and dangerous security breach than the recent purple powder attack on Tony Blair which stunned Westminster earlier in the year.
That attack led to a comprehensive security review which is still on-going, and the erection of a bullet proof screen between MPs and the public.
Yet all those new arrangements failed.
Had the protestors been terrorists the outcome could have been devastating.
Some claimed it was the first invasion of the chamber since Charles I.
It raised questions over precisely how the protesters got into the building and then into the highly secure area where only MPs are supposed to have access.
Flour attack sparked clampdown
Some MPs - including Tory Tim Yeo and Labour's Clare Ward, who was in the chamber at the time of the protest, suggested the demonstrators must have been given access by someone.
Mrs Ward called for an inquiry to find the alleged culprit, adding "we need to know because quite frankly it is unacceptable".
The previous attack on the prime minister had already placed the organisation of security in the Palace of Westminster under intense scrutiny.
But Commons leader Peter Hain has admitted security in the building is out of date and in need of a thorough overhaul and modernisation.
The parliamentary authorities have been taking advice from MI5 in recent months about the terror threat but there is no suggestion so far that such outside authorities should take over security in the Palace of Westminster.
The Metropolitan Police would, however, take over in the event of an emergency situation such as a full-blown terrorist attack.
There are already armed officers placed throughout the building but, understandably, much of the security is secret and unseen and the authorities refuse to discuss it openly.
Protests erupted into violence
What is well-known is that members of the public have to pass through airport-style security checks to get into the Parliament buildings.
It is a long-held tradition, one much prized, that people can then stroll to central lobby to lobby their local MP. However there are a myriad of corridors and rules about who can and cannot go any further - which boil down to a basic rule that no-one can go further unless accompanied by someone with an official security-approved pass.
The latest breach raises fresh questions over whether the internal security really is efficient and well enough equipped to handle the task of protecting the Palace of Westminster.
Some senior police officers are known to believe it is time they were charged with that task.
Similarly there have been suggestions that the security services like MI5 should takeover at least some responsibility for security.
All these issues will now be raised again and it is certain that more draconian security measures will be an almost inevitable consequence of this latest breach.