A year after Dame Eliza Manningham-Buller, then head of MI5, outlined in chilling detail the scale and sophistication of the threat to Britain from al-Qaeda and its terrorist associates, the news from the Security Service remains bleak. Jonathan Evans, her successor, said yesterday that the number of individuals identified as having links with terrorism had risen in a year from 1,600 to at least 2,000. Al-Qaeda was grooming children and teenagers for terrorism and was planning attacks not only from camps in Pakistan but also from new bases in Somalia and Algeria. He said that the Security Service was facing “the most immediate and acute peacetime threat” in its 98-year history.

He said the violence directed against Britain was the product of a much wider extremist ideology, whose basic tenets were inimical to tolerance and liberty, the basis of Britain’s democracy. Intelligence and the security agencies could do no more than contain the threat; there would, he said, be more attempts to cow the country, harm its people and divide its communities.

His assessment is neither new nor surprising, but there remains a reluctance in our society to accept this new reality. There have been attempts in the past year to follow up the 7/7 bombings, with the failed London car bombs and botched suicide attacks at Glasgow airport. Little comfort can be drawn from these crude operations. As Mr Evans noted, the primitive can be just as deadly as the sophisticated. But several factors ought to persuade the public that this is a struggle that not only must but also can be won. The first is that the security services are no longer naive in their risk assessment. Strengthened by a sharp rise in their budget and manpower, bringing MI5’s strength up from 3,000 today to 4,000 by 2011, Britain’s intelligence services are working increasingly with each other and with the police to overcome some of the rivalries.

The second factor is the long-term nature of the counter-terrorist strategy. Just as the Cold War honed skills needed for two generations, so the security services are calling for persistence in preparing for a long struggle. In this, they do not expect to be alone. The public, faith groups, community leaders and the media are crucial to any strategy to defeat those who would corrupt religion for political ends. The crucial issue is trust. Already, Mr Evans said, the security agencies are receiving information daily. He underlined, rightly, that intelligence operated best when it went with, not against, the grain of public opinion. Explaining the purpose of the agency’s work is obviously essential, but so is the support of Islamic groups, which are sometimes tempted to portray the police and intelligence services as intrinsically hostile.

The terrorists’ greatest ally is complacency. Maintaining constant vigilance is costly and time-consuming. The long queues at airports, the repetitive questions and the routine security inspections at public events can appear to the public more of an irritant than a necessary precaution. But they might, at any moment, be the extra obstacle that detects or prevents a would-be bomber. A free society does not welcome a mentality of mistrust, nor one that snoops on neighbours and gossips to agents of the State. But a society will not remain free unless it is intolerant of the intolerant. We need to be assured that the spooks are behaving responsibly. But we also need to ensure that terrorists cannot attack our life and liberty.