Leading article: Intelligence, security and the need to know more

National security is not something to be toyed with. Nor is the safety of airline passengers, or - for that matter - any other members of the public. The very notion of a conspiracy to launch synchronised suicide attacks on passenger planes over the North Atlantic conjures up horrific images. They are images which - in the light of air atrocities that run all the way from Lockerbie through the twin towers to the thwarted efforts of the shoe-bomber, Robert Reid - are all too plausible.

Assuming that intelligence and surveillance reports showed what the police and government ministers have said they showed, there can be little doubt that the authorities had to act. The safety of citizens is one of the chief duties, if not the chief duty, of national governments. Not to have raised the threat estimate to its highest level - "critical" - and not to have introduced the most rigorous checks on cabin luggage for airline passengers would have been irresponsible in the extreme.

In this case, the losses sustained by the airlines, by passengers, by travel companies and by the economy as a whole must be seen as negligible compared with what might have happened if such measures had not been taken. The stock market and sterling may have fallen yesterday at the news of the alleged conspiracy and the airport terrorist alert, and the damage is likely to be felt long after transatlantic flights are back to normal. But this must be weighed against the consequences of doing nothing: "Mass murder", as Scotland Yard put it yesterday, "on an unimaginable scale".

Better safe than sorry is a necessary guiding principle, not least for the government of a country that so recently experienced the death and destruction of the London bombs. Yet there were aspects of yesterday's alert that none the less prompt a certain unease.

It may be that when the Home Secretary warned earlier this week that Britain faced the most sustained period of serious threat since the Second World War, he was speaking with knowledge of the conspiracy targeted by yesterday's operation. To those not privy to this information, however, this massive alert might look like a cynical effort to illustrate the immediacy of the threat. It just so happened, too, that yesterday was the scheduled publication day of a Commons report claiming that troops in Iraq were under-equipped and over-stretched. The bad news was buried. So, too, for the time being, was the gathering call among MPs for Parliament to be recalled over the crisis in the Middle East.

It is possible to have misgivings, too, about the dramatic edge with which yesterday's operations were presented and the grave relish with which Mr Reid seized his chance to take charge. Nor was it altogether consoling to learn of the extensive intelligence co-operation with the United States and the months of surveillance and intelligence-gathering that had preceded this operation. We recall Iraq's non-existent weapons, the notorious episode of the tanks at Heathrow, the shooting of Jean Charles de Menezes and the dawn raid at Forest Gate - to select but a few - as instances of media-manipulation and fallibility. If we instinctively feel we need to know more about the background to this operation before taking it as face value, the Government has only itself to blame.

All that said, we have long argued that the way to combat terrorism is by sophisticated intelligence-gathering and thorough police work, not by the rush to pass repressive legislation that has too often been this government's response. If this operation turns out to have thwarted a plot on the scale outlined yesterday, it will be a triumph of which all involved can be truly proud.