BRITAIN'S security services knew last October that a resurgent al Qaeda and its affiliates had singled out the UK as its priority target for mass-casualty attacks using "home-grown" cells of disaffected Muslims.

With political will leaning towards eventual military withdrawal from Iraq, the terrorist aim was to stage headline-grabbing "spectaculars" designed to allow Osama bin Laden's propaganda machine to claim it had driven British soldiers out of Muslim lands before they pulled out.

Intelligence chiefs also knew that domestic Islamic terrorists were taking a leaf out of the Provisional IRA's training manual by tightening their self-contained cell structure to minimise the risk of infiltration by MI5 or Special Branch agents.

These cells, ranging in size from four to 10 people, have their own "quartermasters" whose duties include obtaining the ingredients for bombs, organising transport, and reconnoitring targets. The only link-up in the terrorist chain is via "mission controllers" who each look after one or more cells, but have no knowledge of their peers if caught and interrogated.

The backchatter on radical websites in recent months provided warning signs something was brewing, leading to intensified domestic surveillance from April. But the attack on Glasgow Airport and the car bombs in London took MI5 completely by surprise, despite a monitoring system which tries to track up to 2000 known or suspected "players".

An intelligence official told The Herald: "They're recruiting young Muslim men born in Britain who have no criminal records or known radical connections. The biggest threat is that of clean skins', the unknown individuals who will be activated for one, specific mission.

"All we can do is rely on snippets of intelligence and pure, blind luck. We were lucky the bombing teams in London and Glasgow were comparatively inept in the execution of the attacks. But there was detailed planning behind them strikes. That's the most disturbing aspect.

"The people in custody are likely to be expendable foot soldiers. Someone else did the detailed work, and is still out there."