A discussion about the airline industry, airline employees, and all things related

A British police source said the plot was believed to involve a "liquid chemical" device.

"Over the last few hours, British authorities have arrested a significant number of extremists engaged in a substantial plot to destroy multiple passenger aircraft flying from the United Kingdom to the United States," the statement released by the DHS press office said.

British police said they had thwarted a plot to blow up aircraft in mid-flight between Britain and the United States and were holding 21 people in connection with the plot. The arrests were made in the London and Birmingham areas.

In response, Britain's security services raised its threat level to "critical" from "severe," the highest of its five ratings, suggesting "an attack is expected imminently."

"Currently, there is no indication ... of plotting within the United States," Chertoff said. "We believe that these arrests have significantly disrupted the threat, but we cannot be sure that the threat has been entirely eliminated or the plot completely thwarted."

"Consistent with these higher threat levels, the Transportation Security Administration is coordinating with federal partners, airport authorities and commercial airlines on expanding the intensity of existing security requirements.

"Due to the nature of the threat revealed by this investigation, we are prohibiting any liquids, including beverages, hair gels, and lotions from being carried on the airplane. This determination will be constantly evaluated and updated when circumstances warrant," the statement said.

"Travelers should also anticipate additional security measures within the airport and at screening checkpoints," it said, and travelers in both countries were told to expect long delays. The changes took effect at 4 a.m. EDT.