Afghan farmers are preparing to harvest a potential bumper opium crop that threatens to fuel the illicit drugs trade in the surrounding region and flood Europe with heroin by the end of this year.

According to western intelligence and customs officials, Afghans planted vigorously in the autumn in areas liberated from the Taliban and now beyond the control of the new administration in Kabul.

British officials believe that unless urgent action is taken militarily to back a crop eradication and aid effort in the Helmand and Nangahar regions within the next four weeks, a large opium crop could be ready for harvest by June.

The assessment is provoking fresh tension between the US and its European allies. British officials - backed by the German, Spanish and Italian governments - want a more vigorous logistical support to be offered to a new aid programme in the poppy growing areas which would include construction work and crop substitution.

For the UK, the political stakes are high. Tony Blair, prime minister, identified the opportunity for eradicating opium production in Afghanistan when justifying British military involvement with the US bombing campaign last October.

But now British officials say that such early optimism was misplaced, with the US government showing little interest in evidence that opium is being cultivated.

"The fact is that on the drugs issue it is showing limited interest and partnership," one official said.

The United Nations security council, of which the US and UK are permanent members, together with Russia, China and France, has not broached the subject in earnest, in part because of Washington's ambivalence.

The UN's drugs control agency, which had been active in combating Afghanistan's poppy production before September 11, has been sidelined by the misallocation of funds by Pino Arlacchi, its former head. This left Europe without a multilateral avenue to pursue the problem, diplomats said.

Meanwhile, intelligence estimates suggest that the current harvest has the potential to produce 4,500 tonnes of opium or 450 tonnes of heroin. About 150 tonnes of Afghan heroin has been entering the European market annually - equivalent to 95 per cent of the European heroin trade.

And in Afghanistan, farmers are getting ready to sow the next season's poppy crop. "Without a crop substitution programme in place, you can't blame the farmers," said one diplomat.