RABAT, July 13 (Reuters) - Morocco's OCP, the world's leading phosphate exporter, will refuse to participate in a South Africa trial over the fate of its seized phosphate shipment from the disputed Western Sahara, the company said on Thursday.

The Polisario Front independence movement, which disputes the territory with Morocco, charges that a 50,000-tonne Moroccan shipment en route to New Zealand was illegally extracted from Western Sahara and went to court to have the cargo seized on May 1.

The case is a test of Polisario's new legal tactics in its long-running conflict with Morocco over Western Sahara, where the two sides fought a war until a 1991 ceasefire but where U.N. talks have failed to yield a lasting political settlement.

A South African court ruled last month that the Marshall Island-flagged NM Cherry Blossom should remain in Port Elizabeth until the case goes to trial, or security for the roughly $5 million cargo is posted.

OCP and Phosboucraa, its subsidiary in the disputed Western Sahara, said they consider the seizure in South Africa to be an act of "political piracy committed under judicial cover".

"They therefore reserve their right to pursue and protect their property rights for this cargo using all means permitted by international law," the OCP statement said.

A Panama court last month dismissed another case by the Polisario to block a phosphate cargo in the Central American country, saying there was no evidence it belonged to the group and that a domestic court was not the venue to judge political matters.

The Port Elizabeth High Court said the core of the case was whether the Polisario movement had made a prima facie case that it owned the phosphate cargo and for an interim order granted in May to stand.

It ordered that the five respondents, including the owner of the ship, the buyer of the cargo and OCP, be restrained from removing the cargo. The Polisario described last month's South Africa court decision as a victory.

(Reporting by Samia Errazzouki; editing by Patrick Markey/Mark Heinrich) ((pat.markey@thomsonreuters.com; +213-661-692993; Reuters Messaging: pat.markey.thomsonreuters.com@reuters.net))