The British military is mystified after what was first announced as a major haul of opium poppy seeds amounted to nothing more than a hill of beans.

British troops came across a bag of seeds -- weighing 1.3 tonnes -- during a major operation near the provincial capital of southern Helmand last week, said a British military spokesman.

The find was originally trumpeted as a big haul of opium poppy seeds. Afghanistan produces about 90 percent of the world's opium.

The fight against opium production is a major element of the battle against the insurgency in Afghanistan because opium is the major source of funding for the Taliban.

However tests on the seeds by the United Nations appeared to show they were in fact mung beans, a perfectly legal if much less profitable crop.

Asked if the suspect kernels were in fact mung beans, Tekeste Tekie, U.N. Food and Agriculture Organisation spokesman in Kabul, said: "There is no question, yes".

The British military spokesman would not confirm the mung bean mystery, saying the true nature of the suspect seeds had not yet been determined.

He said it was a matter for the Afghan army to clarify because they had alerted their British counterparts to the seeds in the first place.

About 9,000 British troops are deployed in southern Afghanistan as part of a NATO-led force. Some 4,000 U.S. Marines launched a major new offensive against the Taliban in Helmand early on Thursday.