THE absence of any prior intelligence about a planned Islamic attack in Madrid is now one of the biggest causes of concern among security chiefs in both Europe and the United States.

As British officials acknowledged for the first time that al-Qaeda rather than Eta, the Basque terror group, was the prime suspect for the Madrid bombings, inquests were under way to see if anything to indicate a bomb plot had been missed.

It is understood that neither British nor American intelligence agencies had been expecting an al-Qaeda attack in Spain and the Spanish authorities were geared up only for a possible Eta strike.

British officials said that the main reason that Eta had been blamed at the start was that it had already tried and failed to bomb the railways, although on a small scale. “This seemed like a second attempt by Eta, but on a much larger scale,” one official said.

The British Joint Terrorism Analysis Centre at MI5’s London headquarters, which operates round the clock, had shared the initial view that Eta was responsible. In spite of the apparent telltale signs of an al-Qaeda attack — notably the well co-ordinated multiple-target approach — Britain and America went along with the Spanish analysis. “The Spanish know more about Eta than anyone else,” one British official said.

The lack of any intelligence to the contrary made it even more inevitable that Eta was blamed, even though it had never before attempted an attack on such a scale. But after the discovery of the “al- Qaeda” videotape and a mobile phone attached to explosives, British security officials admitted: “The wind is now blowing in the direction of al-Qaeda.” The Madrid bombings have reminded the West’s security agencies of an audiotape broadcast by al-Jazeera television station last October, when Osama bin Laden threatened countries involved in the Iraq war. He named Britain, Spain, Australia, Poland, Japan and Italy.

British officials said that the terrorist threat assessment was being updated constantly, but that there were no indications of a likely strike against a British target — although the lack of intelligence before the Madrid bombings was a complicating factor.

However, the officials said that if there were any sign of an imminent attack, all necessary security precautions would be put in place, possibly including the deployment of troops. Geoff Hoon, the Defence Secretary, said that 14 regional contingency reaction forces, made up of part-time reservists, were already set up for crises. London Transport Police is also launching from today a £100,000 poster campaign, warning Underground passengers to look out for unattended bags.

Other countries on bin Laden’s list of targets were also preparing for possible attacks. Poland, which commands a multinational division in Iraq, increased the presence of police and security guards at airports, railway stations and border crossings. Italian security authorities were reviewing the potential threat to transport networks.

John Howard, the Australian Prime Minister, ordered the national counter-terrorism committee to meet today to discuss transport security arrangements.

Apart from the lack of prior intelligence warnings, the method of the bombings was also causing an element of surprise. If it was al-Qaeda, its familiar modus operandi — the use of suicide bombers — was rejected this time. The bombers wanted to survive and used the remote-controlled technology developed by a number of terrorist groups over the years, including the Provisional IRA.

Ironically, it was the choice of a mobile phone strapped to explosives as the triggering device that provided the Spanish authorities with their biggest lead and, directly, to the arrest of three Moroccans and two Spaniards of Indian origin.

Mobile phones have become a crucial ingredient for the modern terrorist — they were used in an attempt to assassinate President Musharraf of Pakistan last year — but they can also supply vital information if recovered from the scene of a bomb attack.

The failure by the Madrid bombers to detonate all ten of the devices was another sign, according to initial judgments, that it was Eta, not al-Qaeda, but after the discovery of the mobile phone and the subsequent arrests, these judgments were being widely reassessed.

With Moroccan counter- terrorists joining the Spanish police in the investigation yesterday, it was clear that efforts are now focusing on an Islamic connection, which raised fears that Britain might be next on the list.