When Airey Neave, the last MP to be assassinated in parliament, was blown up by a magnetic bomb under his car as he was driving out of the underground car park, his murderers boasted they had killed him “inside the ‘impregnable’ Palace of Westminster”.

That was back in March 1979 when the Houses of Parliament were, in reality, far from impregnable. There has always, of course, been a constant police presence at Westminster but even during those years of the IRA and INLA bombing campaigns individual members of the public could always get in and out of the Palace of Westminster with relative ease.

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For most of its history, Downing Street was also accessible to the public, and crowds often gathered at key political moments outside the famous black door guarded only by two police officers, one outside and one inside. It wasn’t until the mid-1980s that gates were added to either end of Downing Street enabling visitors to be vetted before they got to the front door.

But this primitive security measure was not enough to deter the IRA from launching a mortar attack on John Major’s cabinet from a Ford Transit van from the roadway on the other side of Downing Street. The third of the three homemade shells landed in No 10’s back garden, just yards away from where Major was chairing the cabinet meeting. The bombproof windows ensured no ministers were hurt, although four others, including two police officers, were injured.

Since then, two substantial guardhouses, staffed with armed police officers, have been added at either end of Downing Street along with an array of less visible surveillance and security measures.

Facebook Twitter Pinterest Airey Neave. Photograph: Keystone/Getty Images

Access to parliament is also now also rigorously controlled, with airport-style searches, and queues of up to 45 minutes at busy times have become the norm. Movement within the Houses of Parliament is also much more closely monitored as the authorities try to retain the public’s access to their MPs while keeping them safe. Several breaches over the years, mainly by protesters such as Fathers 4 Justice, have ensured that the security systems have been repeatedly tested and tightened.

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The result has been that in the cold language of counter-terrorism both parliament and Downing Street have become “hard targets” in the way they never were when the Irish National Liberation Army killed Neave nearly 40 years ago.

For the hard truth of Wednesday’s tragic attack on Westminster is that the security regime in parliament is so tight that when the attack came it was in the form of stabbing the police officer at the one entrance to parliament without its own search machinery. This is because it is the entrance to New Palace Yard and the same underground car park where Neave was murdered.

The fact is that the attacker in his attempt to spread terror, was reduced to mowing down pedestrians on a crowded Westminster Bridge to tragically fatal effect.

When Jo Cox was murdered in her Batley and Spen constituency on 16 June last year it was notable that she was killed outside a library where she was about to hold a constituency surgery. Only this week, in discussing the rising tide of hate crime and death threats against MPs, especially female MPs, the parliamentary authorities warned that while MPs and their staff felt safe at Westminster, they needed new guidance about how to protect themselves in their constituencies. It is now likely that will be the least of any extra security measures adopted in the wake of this attack.