The United States Department of Homeland Security has raised the terror alert level to Red (Severe) for the first time. The red alert applies only to international commercial flights from the United Kingdom to the United States, while the alert has been raised to Orange (High) for all U.S. commercial air travel. This after U.K. authorities uncovered a highly-developed terrorist plot to blow up U.S. airliners.

The overall U.S. terror alert level remains at Yellow (Elevated), and the United Kingdom government has raised its alert level to Critical—its equivalent of the U.S. government’s red alert.

As a result of this threat, air travelers should expect increased security screening and allow extra time. Passengers on flights out of the U.K. are currently required to check all luggage, and other restrictions may apply depending on your departure/arrival locations.

The plot, which involved smuggling liquid explosives onto flights in terrorists’ carry-on luggage, was uncovered by British authorities who arrested 24 suspected participants. According to DHS Secretary Michael Chertoff, the plot was in its final stages and was ‘suggestive’ of being connected to al Qaeda.