It was just the sort of good news the British military in Helmand needed. Soldiers engaged in Operation Panther's Claw, the huge assault against insurgent strongholds last week, had discovered a record-breaking haul of more than 1.3 tonnes of poppy seeds, destined to become part of the opium crop that generates $400m (£243m) a year for the Taliban.

Ministry of Defence officials more used to dealing with negative stories about the British operation in southern Afghanistan swung into action to extract the maximum benefit from this unexpected PR coup.

A press release hailed the success of the offensive, and armoured vehicles were hastily laid on to allow the media, including the Guardian, to visit the site where the seizure was made, an abandoned market and petrol station that was still coming under sustained enemy fire when the reporters arrived.

Major Rupert Whitelegge, the commander of the company in charge of the area, tugged at one of the enormously heavy white sacks.

"They are definitely poppy seeds," he said emphatically.

Except they weren't. Analysis of a sample carried out by the UN's Food and Agriculture Organisation in Kabul for the Guardian has revealed that the soldiers had captured nothing more than a giant pile of mung beans, a staple pulse eaten in curries across Afghanistan.

Embarrassed British officials have now admitted that their triumph has turned sour and have promised to return the legal crop to its rightful owner.

Dr Samuel Kugbei, the chief FAO technical adviser in the Afghan capital, said: "We have been waiting all day to see these dangerous materials brought from Helmand and now we see that they are just mung beans!"

The pulses also fooled Colonel General Khodaidad, Afghanistan's minister of counter-narcotics, even though the spherical black beans, about the size of small ball bearings, looked nothing like poppy seeds. When shown the mung beans by the Guardian, he said they were a strain of "super poppy".

The beans were introduced into Afghanistan about 10 years ago and have been embraced by farmers as a way of growing a second crop during the year. They are also delicious with rice, Kugbei noted.

If indeed the sacks did contain 1.3 tonnes of mung beans, then they would have a street value of $1,300 – not much, but a major blow to any farmer if the British had followed procedures and destroyed the beans.