There is something wrong with the Government's version of our stunning success in thwarting the planned terrorist attack on aircraft bound from Britain to the United States, bombings that would have "caused loss of life on an unprecedented scale". We are told that, thanks to the brilliance of our anti-terrorist forces, we have avoided another 9/11. Apparently faced with a bombing attack on a number of transatlantic aircraft, "part of the most sustained period of severe threat since the end of the Second World War" (our Home Secretary, John Reid's, words), we have rounded up the "main players" just in time, and they are all in custody.
Their assets have been seized, they are being questioned, and, after a security alert at the highest level and enormous disruption at our airports, we are getting back to normal. But consider this. Instead of celebrating the undoubted skill and dedication of MI5 and the police, the Government should also admit that the affair has revealed that we have been conned for years over our airport security.
We had been led to believe that every possible precaution had been taken to prevent a terrorist carrying a bomb on to a plane in hand luggage. This turns out not to be true. Against the type of attack the group was allegedly plotting, we would have been defenceless.
Original ideas for terrorist outrages are hard to dream up. There is a limit to what is effective, headline-grabbing and yet feasible. Al-Qa'ida's destruction of the twin towers in 2001, by turning civilian airliners into enormous missiles, set a standard that terrorists have been trying to replicate ever since.
They have been frustrated by new anti-hijacking security measures (armoured doors to the flight deck, the banning of sharp objects from hand luggage and passenger profiling). But did no anti-terrorist officer step into a terrorist's mindset to think, "OK, I can't hijack the aircraft and fly it into a building. But I can still turn a plane into a missile by blowing it up from the inside while it's over a densely- populated area of London or New York"? Then the officer would have moved on to the problem of how to get the explosive on to the aircraft. The most effective explosive made from ingredients available to amateurs involves large quantities of agricultural fertiliser. This sort of bomb, once favoured by the IRA, could be quickly ruled out because it is bulky and is hardly the sort of substance one could explain to an airport security officer searching hand luggage.
But there are other explosives that can be made from ingredients available at any chemist's shop. The amounts needed are comparatively small and can be disguised as cosmetics, drinks or medicine. Police say that this is what the group arrested on Wednesday and Thursday was planning to use, taking the ingredients on board separately and then mixing them in the aircraft's toilet.
Amazingly, it turns out that this had been done before. So not only did the anti-terrorist authorities fail to think like terrorists, they could not have taken sufficient note of the earlier event and the lessons it held.
In 1994, Ramzi Ahmed Yousef, a Pakistani linked to al-Qa'ida, carried the ingredients for a bomb on to a Philippine Airlines flight bound for the United States. They were in his hand luggage in innocuous-looking containers, including a bottle of contact lens solution. He mixed them together in the plane's toilet, attached a timer, put the bomb beneath a passenger seat, and then got off the plane at the next refuelling stop.
Soon after take-off the bomb exploded, killing a Japanese businessman occupying the seat and tearing a two foot hole in the cabin floor, revealing the cargo hold beneath. But the fuselage of the plane remained intact and the pilot managed to land safely at Okinawa, with the Japanese the only casualty.
But Yousef's success in getting a bomb through security and on to a plane highlighted serious security weaknesses. While all luggage that will go into a plane's cargo hold is screened for explosives, few pieces of hand baggage are. They go through X-ray machines which can pick up the wires of a bomb's detonator, but X-rays and metal detectors cannot show whether a bag contains explosives - or the ingredients for explosives.
The technology is there - "puffer machines" blow air over passengers and hand baggage to detect whether either have come into contact with explosives. But Peter DeFazio, a member of the US Congress Aviation Subcommittee, says, "We have done nothing at checkpoints to detect the kind of bomb that Yousef designed and which is available to be copied on the internet. That is just unconscionable."
The unanswered question is whether it is possible to make a chemical bomb of the kind Yousef used that would be big enough to bring down a modern airliner. Experts say that it would depend on the location of the device. If it were to destroy structural elements of the plane, or its fuel lines, then it would crash. But most planes could survive if the bomb blew out only the aluminium sheeting of the fuselage.
How much of all this did our anti-terrorism forces know? They must have studied the Yousef case. But then why did they leave it until last Thursday to implement measures to prevent bomb ingredients being carried on to aircraft in passengers' hand baggage? And then announce it in such a dramatic manner?
The most obvious reason is that they received last-minute intelligence that the plot was reaching a climax. And without knowing much more about the plotters' background they were unable to assess how technically competent at bomb-making they might be.
For although the ingredients for a chemical bomb are reasonably easy to obtain, it turns out that successfully mixing them is much harder and more dangerous than it at first appeared. Some of the ingredients may be commercially available but they are too diluted to be of any use in a bomb. Others require chemical refining to purify them. The terrorist could end up blowing his fingers off or setting fire to himself but leaving the aircraft toilet intact. The authorities had to assume, however, that they were dealing with skilled bomb-makers.
As for the dramatic way the news was announced, there is more than just a sneaking suspicion that it suits governments to ramp up the terrorist threat because a sliver of fear makes its citizens easier to lead and control. They can always argue, as the Prime Minister has, that it would be irresponsible not to act on warnings or unverified information - even if these turn out to be wrong - because what if they turn out to be right? In short, we can expect more warnings, not fewer.
And yet we stubbornly refuse to be moved by them. On Thursday, no one panicked. Passengers at airports, interviewed about their reaction, showed a marked reluctance to cancel their flights. The stock market shivered but recovered. On Friday the pubs and restaurants were as crowded as ever. Why aren't we more afraid? One answer is that, although the authorities seem confident that they have thwarted a well-organised and dangerous conspiracy, we have seen previous "threats" crumble away. Since 9/11 there have been more than 600 arrests in Britain to do with terrorism matters. Only 100 of these people were charged and fewer than 20 have so far been convicted. Rightly or wrongly, there is a feeling that the security services and the police, both with increased staffing levels and better funding, want to be seen to be doing their job. Raids and arrests generate good publicity.
A journalist once put it to Dame Stella Rimington, former director-general of MI5, that the threat of terrorism had been overcooked. Surprisingly, she agreed. "You are more likely to be run over by a bus," she said.
"It's difficult for me to say," she continued, "because I've been out of it for 10 years. I've no doubt, though, from what people who do know say, that there are a large number of plots. But at the back of all this, I feel we are tending towards this sense that we must all be 100 per cent safe, and I suppose my feeling is that a better way of presenting it is to say the world is a difficult and dangerous place, and we have to make choices about how much of our civil liberties we want to give up."
So what will happen next? It's a safe bet that the era of easy, carry-on cabin baggage is over. Security checks will get tougher and check-in times longer. We might even have to contemplate CCTV cameras in airline toilets.
And, hopefully, our anti-terrorism forces will adopt one of the few pieces of good advice that the US Defense Secretary, Donald Rumsfeld, has ever given. Once, when seeking to shake up the CIA, he called for a more intuitive approach to anti-terrorist intelligence. Our security services, he said, should "put themselves into the other guy's shoes and think like him".
Phillip Knightley is author of 'The Second Oldest Profession: Spies and Spying in the Twentieth Century'