The most effective way of smuggling explosive liquids onto a commercial airliner without detection would be to use two stable fluids that can be mixed together in a toilet cubicle to make a bomb, chemists said yesterday.

While most conventional liquid explosives are too unstable or easily detected to be suitable, several fluids that are not themselves explosive can be readily combined to trigger a blast.

A prime candidate for this would be triacetone triperoxide (TATP), the explosive used by the July 7 bombers. Its two raw ingredients are both liquids, which could be carried on board in in containers such as bottles of soft drinks or contact lens solution. A small detonator could be hidden in an i-pod or mobile phone, drawing power from its battery.

The two chemicals would be mixed to make TATP, which is a crystalline white powder. Normally, this has to be done at low temperatures to make the explosive more stable, but this would not necessarily be an issue if the aim was to ignite it immediately.

A problem is that the solid has to be dried before it becomes a reliable explosive, and it can be difficult to detonate, as attested by the failure of the attempted suicide attacks on London on July 21 last year. Some formulations, however, would be relatively easy to set off with a simple detonator, or even with a match or lighter.

Andrea Sella, senior lecturer in chemistry at University College London, said: "It would be difficult, but I could certainly conceive of these people taking individual compounds, and mixing them together in the loos. These people are so motivated that they might be nuts enough to set up a chemistry lab in the toilets.

"TATP is something I imagine might be possible to make on an aircraft. You need two lots of liquid, and though these are pretty runny and you’d have to disguise them, it could be possible. Contact lens solution is runny. You then get a solid material that is explosive."

Ehud Keinan of the Technion Institute in Israel, a leading authority on terrorist explosives, said: "It is clear to me that the ‘liquid chemical’ device is an improvised explosive device (IED), simply because all conventional explosives are solids.

"There are a number of ways to make liquid explosives. My guess is that the terrorists have chosen the most dangerous one, the peroxide-based family of improvised explosive such as used in the London bombings last year.

"First, it is very easy to initiate such explosives – there is no need for a detonator and a booster, a burning cigarette or a match would be sufficient to set them off. Second, the raw materials needed for their preparation are readily available in unlimited quantities in hardware stores, pharmacies, agricultural supplies, and even supermarkets. Third, quite sadly, most airports are not yet equipped with the appropriate means to detect those explosives. Practically speaking, there is no efficient way to stop a suicide bomber who carries peroxide-base explosives on his body or in his carry-on luggage."

The practical difficulty of assembling and then detonating such a bomb on an aircraft mean that many attempts would be likely to fail. "I do wonder how easy it would be to do in practice," Dr Sella said. How someone gets up and goes to the loos, with other passengers banging on the door, and does everything right. There would be no guarantee it would work."

This may explain why so many planes appear to have been targeted, to raise the odds of at least one or two successful attacks.

Several commercially available explosives also work on the principle of combining two liquids to ignite a blast.

Some liquid explosives would also be powerful enough to bring down an airliner, but most are too unstable and easily detected to readily evade security checks.

Most liquid explosives, such as nitrogylcerin, are nitrogen-based, and are relatively unstable. This makes their use practically difficult, as they are liable to go off prematurely. The class is also reasonably easy to detect with a technique known as neutron activation analysis, though this is not generally used to screen hand luggage.

Nitroglycerin can be stabilised by combining it with other materials to make a gel, such as nitrocellulose. It would need a larger detonator, which would add to the risk of detection.

There is a precedent for terrorist use of nitrocellulose: it was found in dolls’ clothes in the possession of Ramzi Yousef, one of the masterminds of the 1995 Bojinka plot to blow up aircraft over the Pacific Ocean.

While it would be difficult to blow a plane up completely from within with a small, liquid-based bomb, it could be done by concentrating on weak points such as windows, or by combining several bombs on the same aircraft.

Professor Peter Zimmerman of King’s College, London, said: "Many kinds of explosive can be used to destroy an airplane in flight, because the air pressure in the cabin will add to the destructive power of the explosive. An airliner is a very fast-flying big balloon, and — speaking very figuratively — if the cabin is ruptured and the fuselage skin torn by an explosion at cruising altitude, the aerodynamic force on the rip and the air trying to escape the cabin can greatly multiply the destructive power of a bomb."

If positioned correctly by someone with knowledge of aircraft operating systems, a small device could also sever hydraulic control cables with catastrophic consequences. "You wouldn’t get the spectacular effect of the plane falling apart in the sky, but if it becomes uncontrollable it is going to end up in the sea five minutes later," Dr Sella said.

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