Terror bomb plot intelligence came from MI6's Pakistan operation

Sources with knowledge of the investigation say the intelligence gathered first from abroad and then in Britain before Wednesday's raids, left counterterrorism officials believing there was too high a risk of an "imminent" attempt to bomb targets in Britain and cause mass casualties.

However, subsequent, intensive searches of at least 10 addresses linked to the suspected al-Qaida plot in northern Britain have so far failed to turn up any clear evidence of a terrorist conspiracy, despite the huge resources devoted.

Forty-eight hours after the arrests, sources close to the inquiry say no evidence has been found of bombs, bomb-making parts, precursor chemicals to make explosives, a bomb factory, weapons or ammunition.

The jigsaw of intelligence studied before the Wednesday raids left all agencies - from the police, to security services to Whitehall officials - convinced they had to arrest the 12 people detained. Yesterday they were still in custody. Investigators say the operation is at an early stage.