The alert followed a tip-off from a friendly intelligence agency – thought to be Algeria’s — warning of an imminent al-Qaeda threat.

...A police source said security services had identified two separate Islamist sleeper cells with financial means that were recently “woken up” in France after the arrival of several Islamic radicals from Afghanistan and Pakistan.

Le Monde newspaper yesterday reported that intelligence notes received last week alerted French authorities to an “anti-French focus” by al-Qaida’s North African arm.