A spokesman for the MoD police in the county, said research had shown 18% of drivers had traces of illicit substances in their bodies.

The force is the MoD's own dedicated civil police force of around 3,800 officers, all of whom have full police powers.

They operate at MoD establishments and units throughout Britain.

Doctors in Scotland want the limit reduced

"In support of the annual Christmas drink/drive campaign, MDP in South Wiltshire have a number of officers trained in field impairment testing," the spokesman said.

"We want to identify those drivers who drive whilst under the influence of drugs.

"This is not only aimed at those people taking illegal drugs, but those whose driving is affected by prescription drugs."

Drivers suspected of being unfit, are to be asked to undergo a series of roadside tests to determine their level of impairment.

Road deaths

"Failure may result in their arrest and prosecution," the spokesman said.

Earlier this week doctors in Scotland called for the drink driving limit to be almost halved to make a significant reduction on the number of road deaths.

An Avon and Somerset police spokesman said the force had stopped 4,500 drivers during the first 18 days of December.

They made 149 arrests for failing breath tests or refusing to take the test.