GENEVA, Jan 22 (Reuters) - Thailand won a dispute at the World Trade Organisation on Friday that it had brought against the United States over U.S. anti-dumping measures on plastic bags, a WTO panel said.

The result was a foregone conclusion as the United States did not contest the case, which represents the latest in a series of defeats for a controversial U.S. method of calculating duties on unfairly priced imports, known as zeroing.

"In light of our finding that Thailand has made a prima facie case of violation in respect of the measures at issue, and in the absence of arguments from the United States to the contrary, we rule in favour of Thailand," the dispute panel said in an unusually brief 41-page ruling.

Washington acknowledged that the measures challenged by Thailand were the same as those that had previously been condemned by the WTO.

WTO rules allow members to impose extra duties on goods that are dumped -- imported for less than they cost in the exporting country -- if that hurts businesses in the importing member.

Calculating these "anti-dumping duties" typically involves comparisons of the prices of different batches of imported goods to work out the average difference in price.

In zeroing, the U.S. authorities ignore -- or treat as zero -- examples where the imported goods actually cost more than at home, which results in an unfairly high duty, critics say.

All of the WTO's 153 members except the United States reject zeroing, which has been condemned by various WTO bodies more than 20 times. But the United States says the courts have overstepped their powers and wants the WTO to revise rules to explicitly recognise the practice.

The case dates back to duties imposed by the Department of Commerce in 2004 on retail carrier bags from Thailand. Thailand launched the dispute in March last year.