By Ben Farmer
The province in southern Afghanistan provides two thirds of the world's heroin and more than 95 per cent of that found on Britain's streets.
A British official working in the province claimed in a document released to the BBC under the Freedom of Information Act that 60 per cent of police staff in the area regularly took drugs.
The unnamed official said drug use among the police was "undermining security sector reform and state-building efforts as well as contributing to corruption".
Afghan police are notoriously corrupt, but also bear the brunt of insurgency violence. The casualty rate among police is several times higher than among the army and the daily toll of casualties has been blamed for contributing to the high numbers going absent without leave and turning to drugs.
Another study found both cannabis and opium use was rampant among police across the country.
US police trainers found of 5,320 Afghan police and recruits given nationwide drugs tests, 16 per cent were found to be using drugs.
Gulab Mangal, governor of Helmand, admitted drug abuse was a "huge problem" among police in the province.
A foreign office statement added: "The police are poorly paid, do high risk work and are poorly trained. There are high levels of corruption in the police as well as drug use and supporting counter-narcotics is a key priority for the UK."