Mentoring the Afghan army is a popular job with British troops, many of whom develop close bonds with the Afghan soldiers they serve alongside, but training Afghan police units is a different matter.

The latter are seen as very much the lower end of the quality scale and are widely deemed to be uncertain allies - even though the task of bringing them up to acceptable standards is seen as vital to any Western exit strategy from Afghanistan.

While Afghan army units have undoubted esprit de corps and are recruited countrywide with selection standards for entry, the Afghan police have few if any such standards.

The police tend to be recruited at a local level rather than centrally and, despite efforts to increase their competence through remedial training and improved pay and equipment, they have struggled to emerge as a credible force. ‘Robbers with Uniforms’ is just one of their nicknames among the local populace in southern Afghanistan.

“They’re really a militia, a tribal police whose allegiances are not necessarily to the government or even to the provincial governor. It is normally to their village or tribe or the area they come from," said Captain Doug Beattie, who worked with the AMP during tours of Afghanistan in 2006-07 and 2008.

“Because they’re militia they can be bought and paid off at will. If the government’s paying them they’re reasonably happy. But if they don’t get enough money they’re quite happy to be paid by the insurgency.”

American mentors with an Afghan police unit in southern Helmand this summer estimated that 20 per cent of their charges had some physical disability and over 90 per cent were wholly illiterate. Just 30 per cent showed any motivation to do the job.

A third of police recruits quit each year, many taking their weapon, their training and their contacts straight over to the Taleban.

Ahmad Shah, 27, quit the police in 2003 and joined the Taleban in Wardak. "Back then the salary was only $60 (£36) a month," he said. "We were always getting attacked by the Taleban and I couldn’t visit my family in Jagatai because the Taleban controlled the area. Now I joined the Taleban, I don’t get a regular salary but I get around $300 month and it’s much safer."

While some units, particularly those near the capital, have shown progress under mentoring, at worst the police remain little more than the dregs of Afghan society, a motley crew of drug addicts, petty criminals and former illegal militiamen.

For Afghans travelling through Helmand, running into a police check post is something to be feared. Unless Western mentors are present it will probably result in demands for bribes. Worse can follow, with reports of theft of vehicles, goods, assaults on travellers and even murder by the police. It is common to hear local people compare the police unfavourably with the behaviour of Taleban fighters.

Their local makeup and short training span also makes them vulnerable to local tribal issues and grievances. In the Ned Ali area the police are dominated by the Noorzai tribe, which also controls the huge local opium trade. The kingpin in the local drugs industry, Abdul Raman Jan, was also the provincial police commander until 2006.

Times Recommends

• Google to digitise Iraqi artefacts
• Renault angers parents by naming new car Zoé
• Analysis: most lawless part of Philippines