It is not the first time we have been told that a terrorist attack in Britain is coming, but in the wake of the slaughter in Madrid, the words of the Metropolitan Police Commissioner, Sir John Stevens, were particularly stark.
"There is perhaps an inevitability that some attack will get through," he said at a press conference last week. The Home Secretary, David Blunkett, agreed. So did the Mayor of London, Ken Livingstone, who said: "It would be miraculous if, with all the terrorist resources ranged against us, terrorists did not get through, and ... it would be inconceivable that someone does not get through to London."
The sense that a strike on a major British city was only a matter of time, with London the prime target, was clearly in the minds of the commuters who fled Euston station in near-panic during a security alert the same day, as well as the police, who immediately cordoned off the entire area. In the 10 days since the Madrid attacks, 114 suspect packages were reported to British Transport Police in London and the South-east, a five-fold increase from the 10 days before the bombings.
Public jitters were also reflected in an incident at Knightsbridge Tube station, which was evacuated after a rucksack with Arabic writing on it was found on the platform. A few minutes later, the aggrieved owner called from the next stop to report his bag missing. It seemed that another passenger panicked and threw it out of the train.
Given these fears, why are the authorities making such apparently alarmist statements? For any Londoner who lived through the Blitz or the Cold War, when scores of nuclear missiles were targeted at the capital, or the IRA campaigns from the 1970s onwards, when huge bombs devastated parts of the City and Docklands, explosive devices and incendiaries were left on the Tube and in shops, and six people - including three policemen - were killed by a car bomb outside Harrods, today's reactions might seem somewhat hysterical. Even in the capital, let alone the market towns of Middle England, the chances of any individual being caught up in a terror attack remain minimal.
Sir John, Mr Blunkett and others might appear to be trying to deflect the inevitable criticism when such an attack occurs. But similar warnings were issued when the main threat was from the IRA, and for the same reason: unless the public is recruited, no amount of intelligence or police work, no matter how successful, is sufficient to hold back the terrorists.
As the Metropolitan Police head went on to say, it is his duty to make an attack "difficult, if not impossible". The IRA famously commented, however: "We only have to be lucky once." And it was never as indiscriminate as al-Qa'ida, which scorns warnings and seeks maximum casualties.
The conservative Spanish government lost the election last week because it sought to throw suspicion for the 200 deaths in the Madrid bombings on its own separatists - the Basque ETA movement - rather than al-Qa'ida. Although the spin failed, it was supported by the fact that no suicide bombers were involved, something that had previously seemed a hallmark of al-Qa'ida. However, terrorism experts say that this makes things even harder for those seeking to prevent a similar onslaught here.
"Until Madrid," said one, "it had been the case that no successful 'spectacular' had been mounted in the West since 9/11. It appeared that improved security had forced al-Qa'ida to attack symbols of the West in the Third World - mainly in Muslim countries - such as consulates or military bases. When that became too difficult, they went after softer targets such as the nightclubs in Bali and a hotel in Kenya. But in most cases they still had security to penetrate, so they employed suicide bombers.
"Madrid was different. They did not need to go for a symbol of the West because they were in the West. All they wanted was to kill as many people as possible, and for that they didn't have to blow themselves up. Nor did they need airliners or chemicals or biological agents - just ordinary explosive. If you ask how might it happen here, the answer is: 'Take your pick.'"
Much of the discussion and planning for a terrorist attack in Britain tends to focus on exotic threats, such as what is known as "a CBRN event" - CBRN stands for chemical, biological, radiological and nuclear - and, at least in public, lays the stress on coping with the aftermath of such an attack rather than preventing one. The lesson of Madrid, however, is that radiation detectors at all Britain's ports, one measure being implemented, cannot deal with a more conventional peril. Technology such as X-ray scanners that can penetrate clothing, or electronic devices that can "sniff" explosives, are either too disruptive to normal life or too far in the future to be much help at all.
But there is another troubling outcome of Madrid: Britain's security forces increasingly believe any terrorist attack launched with al-Qa'ida's blessing will be carried out by North Africans, who have been less closely monitored than people from other Muslim regions, such as the Middle East. "The security services are seriously worried, because they believe they only have a handle on about 20 to 25 per cent of radicals from North Africa," said an intelligence source.
Those arrested by Spanish police over the Madrid bombings are almost all North Africans. Key suspects have been revealed to have links with fellow radical Muslims in the UK. Before and since 9/11, MI5 has been behind the disruption of a number of North African terror cells. Last year a number of Algerians were arrested over an alleged ricin plot based in North London.
"Spain's North African émigré community consists mainly of Moroccans, for obvious geographical reasons, and that community provides the cover for terrorist groups linked to al-Qa'ida. In Britain there tend to be more Algerians, so that is the group MI5 pays a lot of attention to," said a source close to intelligence circles. "Algerians are really the only community still large enough to provide cover for an active service unit."
The emergence of North Africans as frontline terrorists is a relatively new and growing phenomenon. Initially they tended to act in support roles, such as providing money and accommodation to active terrorists from the Middle East. Over the past three years Scotland Yard has targeted a number of Algerian gangs for credit card and cheque card frauds, with the cash flow known to be linked to al-Qa'ida sympathisers.
But as it gets harder for al-Qa'ida members, especially those from Saudi Arabia, Afghanistan and Middle Eastern countries, to move freely around the world, British-based North Africans are now best placed to mount any terror attack. "They need to source explosives and bomb-making equipment locally, as transporting across borders is now too risky," said one intelligence source.
Islamist suspects from North Africa are described as "an amorphous target". The majority are previously unknown to the authorities, though a number have trained in the al-Qa'ida camps in Afghanistan. They are just as likely to live in Manchester or Bournemouth as London.
Security officials believe that Jamal Zougam, arrested by Spanish police in connection with the Madrid bombings, has contacts with a number of individuals of North African origin who are at large in the UK. "It is known there are well-established links to Spain and North Africa," said one well-placed source, referring to individuals in Britain. But security sources are reluctant to go into detail, and they dismiss what they say are exaggerated suggestions that the Spanish inquiry has its roots in Britain.
Jamal Zougam has visited Britain at least once with Imad Eddin Barakat Yarkas, alias Abu Dahdah, a senior cleric now held in Spain who has been named as al-Qa'ida's head there. Police believe the pair had a meeting in London with the extremist cleric Abu Qatada, who is detained at Belmarsh prison in south-east London.
Such links were one reason for the decision of European Union ministers last week to appoint a single official to co-ordinate the anti-terrorism work of member states. Improved international co-ordination is essential to head off al-Qa'ida, but most of the effort to prevent an attack in Britain consists of unromantic police work - a poster campaign on public transport, more plain-clothes officers on trains, spreading the message of vigilance in packed public venues such as pubs and clubs.
Another cause for concern is that terrorists may use the Thames and other waterways to move around undetected, so the Met are introducing more high-speed launches to enable them to patrol more efficiently. There are no immediate plans to put on extra officers at Heathrow airport. Security sources say terrorists have looked at attacking Heathrow, but have been put off by the airport's high security.
The fact that terrorists are believed to have used mobile phones to detonate the Madrid bombs has led to calls for London Underground to rethink plans to enable their use in deep tunnels. "Texting is a luxury, security is not," said Simon Hughes, the Liberal Democrat mayoral candidate for London, but the Underground says no decision has been made. There have also been reports that the security services have started "jamming" mobile phones in the vicinity of Tony Blair, to prevent a potential attack on the Prime Minister.
The terrorist threat may have all kinds of longer-term consequences, such as aiding Mr Blunkett in his campaign to introduce identity cards. But the fact is that Britain reviewed its alert state after Madrid, and did not change it: we have been on the second highest level for more than two years now.
The conundrum for the authorities is how to keep the public aware of the danger without sowing widespread panic. Partly because of the IRA, this country has learned to live with the ever-present fear of an attack, but if and when the predictions of Sir John Stevens and the rest come true, that may be no comfort.
The fear factor
The commuter: John Wills, 33, digital mapping specialist from Elstead, near Guildford, Surrey.
"If we do get attacked, then to an extent it's our own fault for invading Iraq, stirring up a hornets' nest. The danger is in the back of my mind but it won't stop me travelling. It makes you vigilant, keeping an eye out for suspect packages."
The tourist: Cyndi Page, dressmaker from Nova Scotia in Canada.
"I'm here visiting relatives. I am concerned. I think the Tube is pretty vulnerable, and I will worry when I take off from Heathrow, because a plane taking off is pretty undefendable from a land missile. I'm more nervous about being in London than in Canada. London being a terrorist target is in my mind."
The war veteran: Roy Shaw, 78, wireless operator and tank gun loader with the Fifth Royal Inniskilling Dragoon Guards in Europe from 1944-45.
"I know what it is like to be scared most of the time. You have to get used to it. I travel on trains and Tubes - there's no point avoiding them, except for the usual reasons of inconvenience."
The psychologist: Dr Michael Reddy, chairman of Independent Counselling and Advisory Services.
"People will settle in their minds that things have become more dangerous than before and they will do something about it. Instead of British politeness, if somebody is behaving eccentrically we should not be afraid to report it."
The countrywoman: Nicky Driver, 27, countryside campaigner from Ledbury, Herefordshire.
"I travel to London two or three times a month. I'm wary of London, but I wouldn't let the threat of terror stop me going because if you do, then they have won. If you alter your daily lives and feel more vulnerable, they've succeeded."
The policeman: Brian Gosden, Chief Inspector of British Transport Police in London and the South-east.
"In the 10 days since the Madrid attacks we have received 32 bomb threats and 114 reports of unattended baggage on the South-east rail network. But there is no reason why people, if they stay alert, should not travel by Tube or train. If they stop, it would be letting terrorists win."