From [1] Sky News. The "critical" level warns of an imminent attack.

Watch this space for updates.

Also, "U.S. Orders Boost in Airport Security in Wake of London, Glasgow Terror Acts," from [2] Fox News:

WASHINGTON — Some U.S. airports will tighten security in response to possible terrorist incidents in Britain, the White House said Saturday.

The United States, however, is not raising its terror alert status, President Bush's spokesman said. "There is no indication of any specific or credible threat to the United States -- no change in the overall security level," Tony Snow told reporters in Maine.

The Transportation Security Administration has taken steps to raise alertness at some airports, Snow said. More TSA agents will be posted outside some terminals, he said.