Existing technology at airports could only find a similar amount of explosive in a squash-court-sized area.

One of the developers, Dr Simon Webster at Leeds University in England, told BBC News Online: "Dogs are very sensitive but they can signal explosives when none are present when they get tired. Our instrument is specific and the chances of false positives are very low."

Semtex is particularly difficult for dogs and existing scanners to find as it gives off very little vapour. But the new device, funded by the UK Home Office, has performed extremely promisingly in tests and will undergo field trials at selected UK ports later this year.

If these are successful, ports and airports may be able to offer greater protection from terrorists.

A cheaper option

The Lockerbie bomb was hidden in a tape recorder inside a suitcase

The machine is hand-held and will cost around £20,000, cheaper than most existing security systems. It is pointed at the suspect package and sucks in a small sample of air. It then shines a green laser on the molecules it contains.

The atoms in different molecules vibrate in different ways. When struck by laser light, every molecule sends back a specific "fingerprint" in light.

In principle, the machine could be used to detect illegal drugs as well as explosives. It could also help forensic scientists when they test for traces of explosives on the hands of suspected terrorists.

"Explosives can be difficult to totally disguise, they can be quite sticky and get everywhere," says Dr Webster. He does not believe wrapping material in clingfilm would be enough to shield the explosives.