June 27 (Reuters) - A U.S. federal appeals court revived an antitrust lawsuit by purchasers of potash that accused seven major producers of running a global conspiracy to artificially inflate prices of the crop nutrient, which is used mainly in fertilizer.

Reversing its decision in September, the 7th U.S. Circuit Court of Appeals in Chicago unanimously said the purchasers may argue that there was a direct, substantial and reasonably foreseeable link between the alleged global cartel and a six-fold increase in U.S. potash prices from 2003 to 2008.

"The allegations suffice, at this stage, to support a plausible story of concerted action," Judge Diane Wood wrote for the eight-judge panel on Wednesday.

Defendants include Canada's Agrium Inc and Potash Corp of Saskatchewan Inc, Minnesota-based Mosaic Co, and four companies from Russia and Belarus: JSC Uralkali, JSC Silvinit, JSC Belarusian Potash and JSC International Potash. Uralkali and Silvinit merged last year.

These companies accounted for about 71 percent of global potash supply when the case was brought in 2008. The United States consumed 6.2 million tons of potash that year, of which 5.3 million were imported.

Stephen Shapiro, a lawyer who has represented potash companies in the case, did not immediately respond to a request for comment.

Potash is used by farmers worldwide, and is also used to make animal feed supplements, glass and soaps.

'REASONABLY PROXIMATE' LINK NEEDED

In the original complaint, purchasers accused the potash companies of ending years of relative price stability in 2003 by conspiring to restrict output and boost prices, citing "parallel" conduct in Brazil, China and India.

Wood agreed with the U.S. Department of Justice that there need be only a "reasonably proximate" causal link between the alleged anticompetitive conduct and higher prices to allow a claim under a 1982 antitrust law governing foreign trade.

She rejected a 2004 decision by the 9th U.S. Circuit Court of Appeals, which oversees several Western states, that suggested higher prices would need to be an "immediate consequence" of the alleged wrongful conduct.

"It is the U.S. authorities or private plaintiffs who have the incentive - and the right - to complain about overcharges paid as a result of the potash cartel, and whose interests will be sacrificed if the law is interpreted not to permit this kind of case," Wood wrote for the 7th Circuit.

Purchasers in the case included Minn-Chem Inc, Kraft Chemical Co and several individuals. Bruce Simon, a lawyer who has represented them, did not immediately respond to a request for comment.

The September decision had been issued by a panel of two judges rather than the normal three, after the third judge had died the previous month. The two remaining judges joined in Wednesday's decision.

The case is Minn-Chem Inc et al v. Agrium Inc et al, 7th U.S. Circuit Court of Appeals, No. 10-1712.

(Reporting by Jonathan Stempel)

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