On Friday afternoon, as MPs and peers were locked in a grand parliamentary row over the abstract principles of liberty and justice, the apparatus of Britain’s secret state stood on its second highest level of alert — “severe (general)”.

The alert has remained enforced since November 2003 when an Al-Qaeda car bomb ripped through the front of the British consulate in Istanbul, killing the consul general and more than 25 others.

It means that an attack is expected on the British mainland but there is no specific intelligence to say where and when. Specialist units of the police, military and emergency services must be prepared to react at a moment’s notice.

The front line in this, Britain’s real war on terror, is not in the corridors of the houses of parliament but some 300 metres along the river inside Thames House, the imposing neo-classical headquarters of MI5, the domestic security service.

On Friday — as the government’s proposed measures for dealing with terror suspects ping-ponged between the Lords and the Commons — it was senior officers from the agency’s G branch who were scrambled to deal with the latest perceived threat to the realm: the impending release into the community of 10 Islamic extremists suspected of plotting terrorism.

Logistics were hammered out. Surveillance teams were readied. Officers were briefed. When such operations go into action Eliza Manningham-Buller, director-general of MI5, typically sends a handwritten report, delivered by courier, to the prime minister.

The terror suspects were being released from three locations — Belmarsh and Woodhill prisons, and Broadmoor secure hospital. Surveillance, curfews, tagging, communication intercepts — MI5 had to get them all in place.

Then on Friday evening as police arrived at Broadmoor to supervise the release of a suspect known only as B, he announced that he did not want to leave. Lawyers say that the Algerian — who has been linked to two fundamentalist north African terror groups and who is known to have bought satellite phones while living in Britain — was then forcibly taken to an empty flat where the door had to be broken down by police to gain entry.

Police, immigration officials, interpreters and lawyers were all involved. The suspected terrorist, who it is claimed has suffered a mental breakdown, will now be monitored under strict rules imposed under the government’s new “control orders”.

It is a year since bombs ripped through commuter trains in the Spanish capital of Madrid, killing 190 and injuring more than 1,500, and for those involved in Britain’s secret war on terror it is no ping-pong match.

Ever since the twin towers were brought down in New York by Al-Qaeda 3Å years ago, an army of spooks and special police units have been striving in the shadows to prevent attacks in Britain.

Unknown to the public at large, a series of terrorist court cases involving more than 20 suspects are under way in Britain — but cannot be reported for legal reasons.

Up to 200 other British citizens and foreign nationals suspected of plotting attacks or supporting terrorism are being monitored.

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