A drill designed to prepare the Commons for a terrorist attack was attended by just five per cent of MPs.

Leader of the House Geoff Hoon told journalists that "about 30 MPs" had taken part and that he "wished there had been more".

With the public being urged to be vigilant, Hoon faced questions regarding the 95 per cent of MPs who had not taken part in the drill.

The former defence secretary said working within the MoD for so long had given him an insight into security, and that he felt the exercise was important enough for him to miss a committee in order to attend.

The serjeant at arms, in charge of Commons security, said that members, staff and the police had rehearsed three different scenarios including a gas attack and an intruder disrupting proceedings.

An evacuation and the chamber being locked with everyone inside were rehearsed.

The drill is part of a tightening up of parliamentary security, particularly in the wake of the London bombings.

Steel barriers have replaced the concrete blocks designed to protect the building from car bombs.

Over the summer recess the security screen in front of the Commons public gallery will be strengthened and extended.

It was first constructed after Fathers4justice campaigners threw purple powder at Tony Blair during prime minister’s questions.

An invasion of the chamber by pro-hunt supporters also highlighted the vulnerability of the Commons; new measures were put in place.