"Anyone who is thinking of rustling trees should beware. They will be rumbled."

Tesco spokesman

Crops of the trees, grown in isolated countryside, are so valuable that they are known in criminal circles as "green gold", and an articulated lorry-load can fetch up to �20,000.

The Tesco guards' arsenal comprises mobile phones to send silent text messages and special night vision goggles.

A spokesman for the supermarket told BBC News Online: "Anyone who is thinking of rustling trees should beware. They will be rumbled."

The guards have already put their army training to the test by building a hide of foliage and tree branches from which they can survey the estate.

'Right skills vital'

The spokesman said the army training was appropriate for patrolling its large tree site.

"Those who steal trees are highly organised. It's very important to get people with the right skills to defend our Christmas trees against thieves. Staying hidden is a very important part of that role," he said.

The squad members will immediately alert the police rather than tackle the intruders.

The spokesman added: "Christmas is a really important time for customers and trees are an essential part of the festivities. We are anxious that no-one is disappointed by rustlers".

One of the guards being deployed is Barry, a 26-year-old who said he could not reveal his surname for security reasons.

He described the Tesco job as like being back in the army.

He said: "After a night's work, we end up pretty cold and wet. We've had high winds the last few nights and everyone has got muddy and dirty."

He limited himself to saying that he and his colleagues had served in the "infantry".