Homeland Security issued its highest terrorism alert ever for commercial flights from Britain early Thursday after Britain said it foiled a massive terrorist plot to blow up several planes.

The AP’s Lara Jakes Jordan reports that terrorists had targeted United, American and Continental airlines. That’s according to two U.S. counterterrorism officials speaking on condition of anonymity because of the sensitivity of the situation.

“The plot was to board international flights, potentially headed to the U.S., with bombs fashioned in a way that they would be in carry-ons, and blow them up in midair,” one intelligence official said.

This official said the terrorists had hoped to target flights to major airports in New York, Washington and California, all major summer tourist destinations.

One intelligence official said the first-ever red alert signaled extreme concern within the government. “We are concerned enough to put the highest wall up we can,” this official said.

Officials said the government has been aware of the nature of the threat for several days, and President Bush was fully briefed.

The alert for all flights coming or going from the United States was also raised slightly, to orange. The government banned beverages, hair gels and lotions from flights, explaining only that liquids emerged as a risk from the investigation in Britain.

Homeland Security Secretary Michael Chertoff issued a statement overnight.

“We believe that these arrests (in London) have significantly disrupted the threat, but we cannot be sure that the threat has been entirely eliminated or the plot completely thwarted,” Chertoff said in announcing that the threat level for flights from Britain to the United States has been raised to the highest “severe or red” level.

“To defend further against any remaining threat from this plot, we will also raise the threat level to high, or orange, for all commercial aviation operating in or destined for the United States,” Chertoff said.

A statement issued by Chertoff said “currently, there is no indication … of plotting within the United States.”