Britain is formally in charge of the international counter-narcotics drive in Afghanistan, which provides 90% of Europe's main heroin supply. But last month the British mission came under attack from the Bush administration for failing to take a more aggressive approach to poppy cultivation.

The US wants more resources poured in and officials to press for destruction even when there is no local political consent and no clear alternative crops are available.

A US congressional committee has been holding an inquiry entitled Are the British counter-narcotics efforts going wobbly?

The differences between the UK and US approaches to combating poppy production burst into the open last month when the British leadership came under attack from Robert Charles, the assistant secretary for international narcotics and law enforcement affairs.

He told Congress: "If we allow this to go on like this for another year we will see institutionalised Colombia-like cartels in the domain where the traffickers coerce the farmers very vigorously to produce."

The Foreign Office minister Bill Rammell is travelling to the country this week to check forecasts of a record poppy harvest this summer.

He admitted: "There have been some differences of emphasis, but it is clear Afghanistan is now at a critical point. The next 18 months will be decisive not just for the poppy production, but the economic stability of the country."

He insisted the US was active in the Central Poppy Eradication Force.

"We are at the start of a pro gramme of eradication through regional governors in the next few weeks. The aim is to eradicate 25,000 hectares of fields, partly though burning," he said. "We are working in a difficult environment, and it was not expected there would be instant results. Experience from Pakistan and Thailand suggests we are where we would expect to be at this stage."

Mr Rammell said he would be meeting police officials fighting production, as well as seeing internal drug rehabilitation programmes.

Privately, Britain believes it is important to win the support of regional governors to undertake effective eradication programmes. But some US voices are demanding an approach that would include military attacks on some suspected heroin storage houses.

Mr Charles said initial figures suggested that predictions of as much as 50% to 100% growth in the overall crop this year were correct, with estimates of production areas ranging from 90,000 to 120,000 hectares, which would bring a world record crop.

Broadly, the harvest begins around now in the south of the country and continues in a counterclockwise direction, ending in the north in September.

A UN survey of farmers' intentions published in February predicted an expansion of production, citing poverty, high opium prices and and access to credit from drug traffickers.