The sense of inevitability does not mitigate the shock and outrage. The perverse point of terrorism is to spread fear by attacking the innocent and assaulting society’s sense of humanity. That the sites selected are so familiar only heightens the sense of violation. It has been 46 months since the atrocities of September 11, 2001, and some 16 months since the Madrid bombings. That passage of time had led some to conclude that the threat from terrorism had disappeared or even that it had never been as intense as ministers and senior police and intelligence officers had consistently stated that it was. After yesterday, any sense of complacency about the character of terrorism, or cynicism about the nature and extent of the threat, must be dispelled.

The objectives of those who commit crimes of this scale are to inflict fatalities on the largest level possible and to spread panic among the population. Yesterday will have triggered fear of a form unknown before for those who were caught up in the explosions, others in transit across the capital and those watching on television. The emotional impact will have been all the greater because of the sickening contrast between the euphoria felt in London on Wednesday and the carnage that followed on Bloody Thursday (7/7) 24 hours later. The city and the country as a whole will have been traumatised by this awful spectacle and that unease will not dissipate quickly.

The Prime Minister and other senior members of the Government will also have been stunned by what took place in London yesterday. Their response to an outrage for which it is always possible to rehearse mentally but never truly anticipate was admirable. It was appropriate that G8 leaders and those of other countries gathered at the Gleneagles Hotel stood together shoulder to shoulder against terrorism. Several of those leaders have experienced terrorism and the collective symbolic retort provided a sign of the resolve that will be necessary in the days, months and years to come.

The moment demanded that political differences be put aside, but there will come a time when it is right to question the preparedness of the police and intelligence services. Charles Clarke, as Home Secretary, managed to convey appropriate information in circumstances of unavoidable confusion. David Davis, the Shadow Home Secretary, was eloquent in his remarks in the House of Commons and offered Mr Clarke his unconditional backing. Ken Livingstone, Mayor of London, while finding himself stranded in Singapore, managed to fulfil his duties at long distance and his extremely personal and uncompromising words directed to those who organised these bombings were well delivered.

It is equally important, on the plausible but unconfirmed assumption that Islamist fanatics were at the heart of this plot, that the Muslim community in Britain is not victimised by others in the population. Whether these terrorists were British citizens or outsiders who have infiltrated our borders, what they have done is also an attack on the principles of the religion whose name they have commandeered and corrupted. It would be wholly wrong to engage in guilt by association. The leaders of various bodies that represent Muslims in Britain have condemned yesterday’s barbaric cruelty. There must be no ambiguity in that message and it cannot be repeated by these organisations too often or loudly.

There may be a few people inclined to make a link between the deaths in London and the intervention in Iraq. This is utterly flawed thinking. Al-Qaeda and its subsidiary branches began their sadistic campaign more than a decade ago and they did not require the removal of Saddam Hussein from power in Baghdad as an extra incentive. London was not targeted because British troops are in Iraq or because of Tony Blair’s alliance with the Bush White House. Rather, London was attacked because these extremists want to ignite a “holy war” between themselves and democratic societies.

In the light of that warped ambition and the obvious capacity of small numbers of dedicated extremists to recruit individuals, obtain explosives and execute acts of terrorism, national culture has, unfortunately, to change. There is a need for much more personal vigilance for the population at large, particularly when travelling, and not only in London but throughout the nation. Numerous public information campaigns have been launched since 9/11, but until yesterday there will have been many who did not spare the posters in prominent places enough thought. It would be reckless to presume that such bombings will not be repeated in Britain.

That vigilance will have to be rebuilt in the public sphere as well. The absence of a terrorist incident in Britain since 2001 has, quite understandably, prompted calls for a rebalancing between counter-terrorism and civil liberties. It is, of course, vital that civil liberties are not trampled in a rush of draconian legislation. It has to be conceded, though, that new legal powers may be required to combat terrorism and that will have an effect not only on those suspected of terrorism sympathies but the broader public. Ministers should not reach for the statute book with undue haste, but citizens must also understand that the security situation today is hardly normal.

Mr Blair declared forcefully yesterday that “those engaged in terrorism” should “realise that our determination to defend our values and our way of life is greater than their determination to cause death and destruction to innocent people in a desire to impose extremism throughout the world”. This is a sentiment that will be endorsed throughout Britain. Despite the shock, horror and outrage, the calm shown in London was exemplary. Ordinary life may be inconvenienced by the spectre of terror, yet terrorism will not force free societies to abandon their fundamental features. An attack was inevitable. The casualties were dreadful. The terrorists have only strengthened the resolve of Britain and its people.