Several different kinds of liquid explosive may have been involved in the terror plot, according to experts.

They are not difficult to obtain or make from raw ingredients, and can easily be hidden inside innocent-looking bottles or cans, said explosives technician Dr Sidney Alford.

One of the chemicals - nitromethane - is used as a fuel for model aeroplane engines.

The best known "liquid bomb" is nitroglycerine, commonly thought of as a deadly material that will detonate with the slightest movement.

This idea is a myth, said Dr Alford, chairman of the explosives company Alford Technologies.

In fact nitroglycerine, kept properly, is relatively stable and will not explode unless "you whack it quite hard," he said. Usually mixed with another material, such as nitroglycol, it would have to be ignited with a detonator.

As well as powering model planes, nitromethane is used commercially, both as an explosive and an industrial solvent.

"It's quite easy to get hold of in relatively small quantities," said Dr Alford.

To be used in a bomb, nitromethane would first have to be combined with another sensitising substance.

A similar alternative - nitroethane - is less well known but equally as effective. There are fewer restrictions on obtaining and transporting this chemical.

"Everyone in the business knows that nitromethane is an explosive, but many people, including some in the police and security services, haven't cottoned onto nitroethane yet," said Dr Alford.

A fourth candidate - methyl nitrate - is unusual in that it will explode as soon as it is combined with another sub-stance. It does not need to be ignited by a detonator device.

"It can reliably be caused to detonate by mixing it with ingredient X," said Dr Alford. He was not willing to reveal the name of the activating chemical.

He added: "The fact that you don't need a detonator would be a great advantage. Methyl nitrate has been used as a component of an anti-personnel mine. When you step on the mine, you break a container which causes the substances to mix."

Other substances belong to a group of explosives from the United States called Astrolites. These are based on hydrazine, which is used to make rocket fuel. Several different varieties exist, all of which are very poisonous and require detonators.

Liquid explosives would not necessarily be picked up by "sniffer" type security scan-ners if placed in carefully sealed and cleaned containers, said Dr Alford. They could easily be disguised and had a surprise element.

"Most people associate explosives with either solid materials or gases," said Dr Alford. "You don't expect an explosive to be liquid. If it's in a baby's bottle, or a clearly labelled bottle of gin or whisky, or cough mixture, how many security staff are going to question it?"