The study shows that 10,000 tonnes of marijuana worth $35.8bn (£18.4bn) is grown each year; the street value would be even higher. This dwarfs the $23bn-worth of corn grown, $17.6bn-worth of soybeans and $12.2bn-worth of hay. Marijuana is the biggest cash crop in 12 states, with the value of pot grown outstripping peanuts in Georgia and tobacco in North and South Carolina. In California, the biggest producer, it is worth $13.8bn.

The report, Marijuana Production in the US, by DrugScience.org, which wants marijuana to be reclassified, says the drug is listed as a Schedule 1 drug, deemed to have no medicinal value and a likelihood of abuse. Other such drugs include heroin.

The author, Jon Gettman, says the figures show the war on drugs is not working: "Illicit marijuana cultivation provides considerable unreported revenue for growers without corresponding tax obligations to compensate the public for the social and fiscal costs related to [its] use."

His suggestion that the crop be legalised and taxed was rejected by the White House Office of National Drug Control Policy, which pointed to countries with large drug cash crops, notably Colombia and Afghanistan.

The report says output in the US has grown tenfold in the last 25 years.

The boom in domestic production has in part been fuelled by tougher border controls after 9/11. As smuggling from Mexico has become more difficult the drug cartels have moved their operations into the US, often creating plantations in remote national park land.