SCOTLAND YARD is braced for a fresh wave of possible terrorist attacks against public transport after a group of suspected Islamist suicide bombers were arrested in a secret security operation at Gatwick airport.

Six Pakistani men were held under anti-terrorism laws 10 days ago after they flew in from Barcelona. The arrests were prompted by a tip-off from the Spanish intelligence services after the discovery of a suspected Al-Qaeda terror cell in the city. The cell is alleged to have planned to detonate suicide bombs on the Barcelona Metro. The Spanish warned a similar attack was being planned here.

The six Pakistanis were taken to Paddington Green police station in west London and were questioned by detectives from the Yard’s counter-terrorism unit. After being held overnight they were driven under police escort back to the airport and escorted onto a flight back to Pakistan.

Soon after the arrests MI5’s Centre for the Protection of National Infra-structure warned of a possible terrorist attack on bridges, tunnels and the Channel tunnel.

The Gatwick arrests followed an antiterrorist operation in Barcelona in which 12 Pakistanis and two Indians were held for allegedly planning bomb attacks. It has been reported that among the cache was a quantity of triacetone triperoxide, a comparatively cheap explosive used by suicide bombers in the Middle East.

At the time of the Barcelona arrests counter-terrorism officials played down reports of any imminent threat to Britain. But it has since emerged that Spanish intelligence did receive information about an attack against Britain’s public transport system.

According to El Pais, the Spanish newspaper, a member of the Barcelona cell told the Spanish civil police that it was planning attacks not just on Barcelona’s underground system but also against public transport in Britain and other European countries.

The informant is said to have told police that pairs of suicide bombers planned to strap explosives to their bodies and blow themselves up on the rail and bus networks in Britain, France, Germany and Portugal.

Referring to the arrests of the Pakistanis at Gatwick, a senior British official said: “The intelligence from Spain was that there was to be another wave of attacks on the way to us after the attempted attacks in Barcelona. When it was followed up it led to this lot.”

Patrick Mercer, the Tory MP who last year helped to prepare a report on terror threats to crowded places, said: “This intelligence shows the breadth and span of international terrorism and anybody who thinks that the threat has either gone or is in abeyance is in cloud-cuckoo-land.”

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