Western intelligence sources say they have uncovered a plot by al Qaeda to carry out a series of coordinated attacks in Western Europe.
According to information leaked to journalists, officials believe the plot, discovered in its early stages, would involve seizing civilians in a number of cities in Britain, France or Germany and murdering them.
Intelligence agencies on both sides of the Atlantic have been tracking one of the most serious al Qaeda attack plans in recent years, the BBC reports.
The idea is thought to have moved from the aspirational stage to actual planning and Western security agencies may have been hoping to keep their knowledge of it out of the public spotlight for longer so criminal evidence could be gathered, but initial details were leaked to the American press.
In the United Kingdom, the national threat remains at severe, where it has been since January, meaning a terrorist attack is thought highly likely.
Government officials say there are no plans to raise it to the highest level of critical, and they do not expect to see an imminent wave of arrests.
France and Germany are both on a heightened state of security alert.