In some of the tightest restrictions ever placed on air travellers, Britain's Department of Transport said on Thursday passengers travelling from British airports could carry only a clear plastic carrier bag.
The restrictions came after British police said they had arrested 21 people in connection with a plot to blow up planes flying between Britain and the US.
Michael Chertoff, the US Homeland Security Secretary, said the plot involved attackers carrying bomb components made from liquid explosives and detonating devices disguised as beverages, electronic devices or other common objects.
British police have not confirmed media reports that liquid explosives were part of the plot.
Passengers travelling from London's Heathrow airport were not allowed any hand baggage except for articles placed in transparent bags, and faced restrictions on fluids.
Those travelling with an infant were required to taste the contents of bottles of baby milk.
US air travellers were also ordered to throw out their suntan lotion and water bottles and waited hours in ever-lengthening lines on Thursday as airports tightened security and delayed flights.
In major US airports, guards armed with rifles stood at security checkpoints, and passengers were met by signs warning that all liquids were now banned from carry-on luggage.
Airports delayed flights following
the terror plot discovery
The travellers poured out liquids, opened their bags for inspection and endured long waits on Thursday as airports heightened security and some flights were cancelled or delayed.
The airlines targeted included United Airlines, American Airlines and Continental Airlines Inc., two counterterrorism officials said Thursday.
Security workers were opening every carry-on bag that passed through one terminal at Baltimore/Washington Airport and all the flights there were delayed.