British Home Secretary John Reid says Britons must remain vigilant even though the government has lowered the terror threat level.
Authorities Monday scaled back the threat level from "critical" -- meaning an attack may be imminent -- to "severe."
Reid said the threat of an attack remains, as police investigate last week's failed plot to blow up 10 passenger jets using liquid explosives.
A British court Monday gave police permission to continue holding 23 suspects without formal charges for at least two more days. Seven other suspects are detained in Pakistan.
British and U.S. airports have eased some of the strict security procedures imposed after the bombing plot was revealed last Thursday.
The new rules have caused massive delays and cancellations of flights leaving British airports. Officials at London's Heathrow Airport say they expect about 20 percent of Tuesday's flights will be canceled.
British authorities will now let passengers carry on a single bag the size of a laptop computer. But a ban on all liquids other than medicine and baby food remains.
U.S. authorities say passengers can now carry on small amounts of medicine, including insulin and certain types of gel.