Kim Sengupta: At times like this, over-reaction is the sensible course

The IRA message 'you have to be lucky all the time, we have to be lucky just once', has never been so relevant

The cancellation yesterday of a BA flight to Washington, in one of the busiest holiday periods of the year, came amid revelations that three Air France planes were grounded over the Christmas holidays because the FBI mistook a Welsh insurance agent, a 10-year-old boy and an elderly Chinese woman for al-Qa'ida terrorists. The office of the Mexican president, meanwhile, was letting it be known the US intelligence that led to the aborting of two of their services was "unconvincing".

All this has led to the expected accusations of over-reaction by the Bush administration. Two years - and two wars - after 11 September, the Americans continue to jump at shadows, imagining terrorists everywhere. The ever-supine British, it is said, are ready to give in to this paranoia, readily acquiescing to demands for armed sky marshals. It is left to plucky Denmark and Sweden to resist Washington's demands for guns on airliners.

This view, however, is highly simplistic. In the current security climate, over-reaction is preferable to under-reaction. The stakes are too high. The IRA message to Margaret Thatcher after the Brighton bombing - "You have got to be lucky all the time, we have got to be lucky just once" - has never been so relevant, even if the threat comes from another quarter.

As the Hutton inquiry showed so clearly, there are differences within the intelligence community on how espionage information is interpreted. The sole function of a highly paid CIA threat analyst, goes the quip, "is to look at al-Qa'ida and say they are a threat".

But the current threat was not dreamt up. Over the last month, intelligence agencies in the West, the Arab world and Asia have been picking up talk of an attack. The snippets have come from telephone intercepts by GCHQ in Britain, the National Security Agency in the US and other listening posts, as well as human intelligence. Such "chatter" preceded the bombings in Turkey, Tunisia, Saudi Arabia and Bali, and indeed 11 September itself. Although the terrorist groups are becoming increasingly proficient at anti-surveillance techniques, a major operation involves a certain amount of seepage.

Pieced together, the jigsaw of information from various countries indicated a desire for an attack using aircraft - and, according to the Americans, flights to Los Angeles, Washington and New York are particularly targeted. But the warnings have not been just about possible hijackings. The closure of the British embassy in Lima and the advice for British nationals not to travel to Saudi Arabia also followed intelligence about possible dangers. It is believed that the group that has twice blown up foreign housing complexes in Riyadh in the last year is ready for another suicide bombing.

In Latin America, Hizbollah, Hamas, and the Islamic Group of Egypt maintain links with organised crime in Argentina, Brazil and Paraguay. Members of these Islamist groups are said to have established themselves in Peru as a precursor to an attack on a high-profile Western target.

Tom Ridge, the US Secretary for Homeland Security, has declared that al-Qa'ida intends to "match" or even "outdo" 11 September. This may be an attempt to justify the disruptions the public are facing in their daily lives, but that criticism does not to stand up to scrutiny.

Part of the problem faced by the security agencies is that following the Afghan war, Islamist terrorism has become much more unstructured. Bin Laden and his lieutenants do not hold board meetings to decide on targets. The emerging pattern is that what is left of the al-Qa'ida leadership - two thirds of them having been killed or captured - request attacks at suggested times and regions, but leave details and exact scales to local groups.

The other problem the US in particular faces - and which led to the embarrassing misidentification of the Air France passengers - is the lack of an accurate, current, list of terror suspects and their associates. Two years and four months after 11 September, and the creation amid much fanfare of the Department of Homeland Security, US agencies are still forced to rely on 12 different databases, some of which are out of date and technically incompatible with each other. A Terrorist Screening Centre has now been formed to integrate the lists, but the process may take years.

But whatever the shortcomings of the present system, there is no realistic alternative to making the best of it. Tony Blair's decision to stand "shoulder to shoulder" with George Bush means that British citizens, along with Americans, are prize terrorist targets. Only very good intelligence, and some lucky breaks, has prevented a major atrocity over here.

France has had its share of Islamist attacks in the past. But, with its opposition to the Iraq invasion, it is not so much in the firing line at present. Yet long before Washington's request for sky marshals, armed French special agents had been flying undercover on a number of flights to the US.

France has also agreed to supply US officials with passenger manifest lists of any flights that warrant scrutiny at least an hour before take-off, rather than while the plane is in the air, as is the common practice, so that vetting of passengers can take place before they board. As officials in Paris point out, these measures simply reflect the reality we face in an increasingly dangerous world.

k.sengupta@independent.co.uk