Shocking revelations of murder, sexual abuse of young boys, unarmed civilians being shot at, police officers high on drugs, and routine kidnaps and extortion are exposing the true state of Afghanistan's security forces in Helmand province.

An investigation has revealed how Afghan forces running bases that British soldiers fought to secure are barely able to function – let alone pose a challenge to the Taliban.

More than 100 British soldiers lost their lives in Sangin before handing over to the Americans in 2010. Foreign troops are due to leave their combat role in Afghanistan at the end of 2014 – passing security to the Afghan government.

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But the despair of the US marines advising Afghan forces is laid bare in scenes being broadcast on BBC1's Panorama tomorrow evening.

It reveals how three boys were shot dead escaping from police commanders who were sexually abusing them. Major Bill Steuber meets the deputy police chief, who claims the children want to stay at the bases and "give their bodies at night". Eventually he agrees to take action, but the operation to rescue the other "chai boys" used as sex slaves does not happen.

Major Steuber angrily gestures to a nearby Afghan patrol base commander :"who we know is kidnapping boys and sexually molesting them, robbing the people. He treats the people of Sangin like a piggy bank that he can just shake and rob and, you know, that's really difficult, you know. How do you work with a guy like that?"

Ben Anderson, the reporter embedded with the US marines, has been visiting Afghanistan since 2007 and said: "The police are behaving like the worst criminals imaginable." He added: "Even going out there with very low expectations I was still shocked at how widespread the abuse is and how it's not really being tackled."

Reacting to the investigation, Colonel Richard Kemp, a former commander of British forces in Afghanistan, cautioned that Afghan forces "will not be ready to take on the insurgency alone when coalition forces leave. Instead many areas of the country will suffer brutality, abuse and corruption as well as surreptitious collusion with insurgents."

Afghan forces are seen under the influence of drugs, indiscriminately firing into the air without regard for civilians, and US soldiers discover four men kidnapped by police and used to barter with the Taliban.

The scenes are in stark contrast to the Defence Secretary Philip Hammond's assurances during a visit to Helmand earlier this month that transition is "on track" and that the Afghans "will be able to maintain security as the Isaf draw down."

A Ministry of Defence spokesperson said in a statement: "We are unable to comment on the specific incidents raised by Panorama" and insisted the Afghan security forces are a "professional and capable force" in the "vast majority of cases" but admitted "real challenges remain across Afghanistan, including proper adherence to the rule of law and respecting human rights".