By Jason Neely and Michael Holden

LONDON, Aug 10: A UK bomb plot foiled today echoed one planned a decade ago in Asia and lays bare the threats airlines still face despite heightened vigilance since the deadly hijackings of 2001, security analysts say.

UK and U S authorities said several airliners set to fly from London to the United States were to be blown up by bombs which suspects planned to carry on board in hand luggage and ordered passengers not to carry liquids on board.

Dr Peter Neumann, director of the Centre for Defence Studies at London's King's College, said the plot bore many similarities to one in 1995 by Ramzi Ahmed Yousef, convicted of planning to blow up the World Trade Center.

''He actually planned to blow up 11 planes over the Pacific with a liquid explosive,'' Neumann told Reuters. ''This is almost exactly the same type of plan we see emerging in this particular instance.'' Department of Homeland Security Secretary Michael Chertoff in raising the U S security level on aviation to its highest level for the first time, said plane passengers would be banned from carrying on board any liquid other than medicine or baby food.

He said the step was being taken to give authorities time to change screening procedures.

On-board armed guards, reinforced cockpit doors and sweeping bans on carrying even finger nail clippers on to planes are some of the measures taken to safeguard air passengers since the September 11 attacks in the United States.

Scanners that sniff for weapons and explosives in hand luggage as well as in bags carried in the bellies of planes have also been improved but analysts say the sheer volume of traffic faced by the world's airports makes the security challenge a daunting one.

''The sad truth is in order to bring in a level of security at international airports that would be enough to give you a 99 percent chance of thwarting any attempt would mean such enormous delays and costs that it would bring international air travel to a halt,'' said Alex Standish, editor of Jane's Intelligence Digest.

London's Heathrow, Britain's largest airport, saw more than 67 million passengers last year. Not only airport operators but airlines have been footing the bill for the new equipment to ensure safe flying. ''The cost of security is about 5.6 billion dollars in addition to what they were spending prior to 2001,'' said a spokesman for airlines group IATA.

Still, the threat from those determined to pierce such security goes on, forcing advances in security on planes, at airports and around them, analysts say.

''You have to detect things very rapidly and you don't always know what it is you're looking for, unlike working in a lab,'' said Jon Salkeld, a managing director at UK technology firm QinetiQ.

''It's unlikely we're going to have a magic box in the foreseeable future which thwarts all threats.'' Since 2001, a series of incidents has led to additional layers of security at airports, as seen with checks on footwear following the thwarted ''shoe bomber'' incident in December of that year.

But bombings aboard trains in Madrid in 2004 and Bombay last month underscore another problem - bombers can simply change their choice of targets.

''It's a never-ending game of thrust and counter-thrust, you can't ever say that anything is 100 percent safe, you can't stop people if someone is that determined,'' said Chris Pope, head of intelligence at RUSI, a London-based defence think-tank.

Analysts said one reason why threats faced by air travellers continue to emerge is that those determined to cause loss of life can succeed with very small amounts of explosive, even mixing it using smuggled chemicals onboard.

''To bring an aircraft down you don't need an enormous amount of explosive because the issue is whether you can compromise the integrity of the aircraft structure,'' said Standish of Jane's Intelligence.

UK police said 21 people had been taken into custody and searches were ongoing following an investigation which had taken months.

Stepped up security checks on all passengers and luggage prompted British Airways and other carriers to cancel some flights as delays were seen at airports across the UK.

REUTERS