Less than half of the fledgling Afghan National Army's 32,000 British and US-trained soldiers have chosen to re-enlist after three years in uniform, according to figures obtained by The Herald.

The 42% retention rate means Nato troops will have to shoulder the burden for security, and the casualties that go with it, well beyond the three-year mission authorised by the UK government - and due to expire in 2009.

Plans to train and field a 70,000-strong national force are already running two years behind schedule, and only about 20,000 local soldiers can be located with their units at any given time. Although desertion rates have slowed from the 50% recorded in 2005, the Afghan troops are unhappy with rates of pay, £40 a month for new recruits, as well as lack of basic equipment and poor logistics.

Many also claim their families have been threatened by the Taliban and told they will be killed unless their relatives leave the government army. The US has spent almost £600m on training since 2003 and British forces in Helmand have embedded "mentoring" teams with Afghan units, although no figures are available for UK cost of training.

A Ministry of Defence spokesman said: "Members of the Afghan National Army are required to re-enlist after an initial three-year contract. To date, approximately 42% have done so. The US-led training mission has already identified measures to improve the level of re-enlistment, including delivery of more Nato mentoring teams for Afghan units and is establishing plans to implement a more sustainable force generation cycle."

A UK officer who has worked with the Afghans told The Herald: "Their own army lacks everything from body armour and weapons to a guaranteed supply chain. And there is a lot of corruption. It's just the Afghan way.

"Everybody takes a cut of things from the top down. The national police are even worse. Some of their officers have sold their vehicles and even their weapons. They also prey on civilians, imposing their own taxes at checkpoints."