By Martin Patience
BBC News, Kabul
Afghanistan is the source of 90% of the world's illicit opium
Sixty per cent of the Afghan police in the country's southern province of Helmand use drugs, it is claimed.
The estimate, made by a UK official working in the province, was contained in emails obtained by the BBC.
International forces are fighting a fierce counter-insurgency campaign against Taleban militants and other insurgents in Helmand.
But British officials are clearly worried about the reliability of the Afghan police.
"We are very concerned by the levels of drug abuse among the police," the British Foreign Office said in a statement.
"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," it said.
Meanwhile, 700 British and Afghan troops were involved in raids on four factories in Helmand, seizing heroin and drug-making chemicals with an estimated street value of more than £50m.
Defence Secretary John Hutton praised the troops' bravery and said the seizures in Helmand province would starve the Taleban of funding.
The training of the Afghan security forces has been a central plank in the international community's strategy to help stabilise the country.
The unnamed British official, however, wrote in an email to the Foreign Office that drug use was "undermining security sector reform and state-building efforts as well as contributing to corruption".
Helmand province produces almost two-thirds of the world's opium, which is used to manufacture heroin.
The provincial governor, Gulab Mangal, told the BBC that drug use was a "huge problem" amongst police stationed in the province.
He added that steps were being taken to tackle the issue and that "at least 10" police officers had recently been dismissed after failing drugs tests.
Drug use amongst the police is not just confined to Helmand, but is a nationwide problem, according to the emails, obtained by the BBC under a Freedom of Information Act request made to the Foreign Office.
Of 5,320 Afghan police and recruits tested by US-led police training programmes across the country, 16% were found to be using drugs. The majority of those who tested positive had used cannabis or opium.
Analysts say that the drug problem in the police is higher in the southern provinces where drugs are readily available - in Kandahar province, which neighbours Helmand, 38% tested positive.
Police work in these areas is also highly dangerous and low paid - reasons, analysts say, for widespread drug use.
"The police are constantly under threat from the Taleban," says Abdul Ghafoor, director of the Regional Studies Centre of Afghanistan, a think-tank based in Kabul.
"To escape from the psychological pressure they often turn to drugs."
But Mr Ghafoor insists that it is vital for Afghanistan that the police act within the law.
"The police are responsible for controlling drug trafficking, but if they become addicts who will control it?"