A special microdot-filled fluid normally used by motorbike and car manufacturers could persuade the thieves to stay away.

The fluid, similar to nail varnish, contains thousands of dots the size of pin-heads, each bearing a unique PIN and telephone number.

The dots, almost invisible to the naked eye, apparently work as well on trees as machinery.

Former soldiers protecting valuable trees

Wiltshire Police spokesman David Taylor said: "Christmas trees sell for at least £10, and a gang could easily take a chainsaw to 200, fill a truck with them and make £2,000 for a night's work.

"The trouble is it is very hard to identify stolen Christmas trees, as they all look similar.

"These microdots mean each tree has a unique code relating to where it was grown in several places on its bark.

"If our officers spot trees being sold in unusual places, car parks, or boot sales for example, they can scan them to check where they have come from."

Alpha-Dot's managing director Nick Dearsley said: "It seems to work really well, as the dots are almost invisible to the naked eye.

The ex-soldiers were employed by Tesco

"Thousands of them will fit into a very small space, yet the police only need to find one to identify where a tree has come from.

"I suppose it does sound quite odd, but it isn't the weirdest use anyone has found for Alpa-Dot. We have sold bottles to people who wanted to use it to mark their tortoises.

"And there was even one chap who painted it onto his parrots' beaks so he could identify them if they were stolen."

Christmas trees, grown in isolated countryside, are so valuable they are known as "green gold" in criminal circles - an articulated lorryload of them can fetch up to £20,000.

Two years ago, supermarket chain Tesco employed three ex-squaddies equipped with camouflage gear and night-vision goggles to cut Christmas tree rustling.