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.