BANGKOK, Thailand — A potential trade agreement between Thailand and the United States could derail this country's production of inexpensive AIDS drugs and imperil the future of an anti-HIV program that is widely considered a model for countries throughout Asia, the group Doctors Without Borders said Monday.

"If the Thais sign such an agreement, they will have to close down their generic drug production," Paul Cawthorne of the Belgium-based group told a news conference. "Trade rules are the biggest threat" to the fight against AIDS, he said.

Thailand is one of the few countries — others include India and Brazil — that manufacture generic versions of anti-HIV drugs developed by U.S. manufacturers.

The country began researching manufacturing techniques for the drugs in the early 1990s and was preparing to market a generic version of the drug didanosine when Bristol-Myers Squibb Co., the drug's manufacturer, served notice that it held a valid patent on the drug. The Thai government was ready to accede, but Doctors Without Borders urged it to fight the claim.

The following year, Thailand's Central Intellectual Property Court ruled the patent invalid in Thailand, paving the way for the country to begin large-scale drug production.

That decision was, in effect, reinforced last September when the World Trade Organization agreed that poor nations could ignore patents in times of national health crises.

A free-trade agreement — meant to greatly expand business exchanges between the United States and Thailand — would incorporate language reinstating the patents, in an effort to protect U.S. drug companies. The Bush administration has also argued that the generic versions of the drug are potentially unsafe and that they are not as effective as the branded versions, a claim most experts dispute.

The reinstatement of patents was part of a similar agreement signed last year by Singapore. Brazil has refused to sign an agreement because of the provision.

Cawthorne, head of Doctors Without Borders in Thailand, urged the Thai government on Monday to follow Brazil's example lest it upset its thriving generic drug industry.

In March 2002, the Thai Government Pharmaceutical Organization began producing a single pill that contains the three drugs recommended by UNAIDS for first-line treatment of HIV infection: stavudine, lamivudine and nevirapine. The pill reduced the monthly cost of treatment for an individual from an estimated $750 to $30. The government now plans to provide it free to 50,000 Thai citizens.

Thai Prime Minister Thaksin Shinawatra said Sunday at the opening of the 15th International AIDS Conference here that the government would spend $20 million to provide about 40,000 of those patients with drugs and that the rest of the money would come from the Global Fund to Fight AIDS, Tuberculosis and Malaria. He also said Thailand would soon begin exporting the drugs to neighboring Cambodia, Laos and Myanmar.

Thaksin said Thailand would also offer its manufacturing technology to Africa in the near future.

The high cost of AIDS drugs has been a continuing theme at the meeting, whose title this year is "Access for All."

On Monday, activists shut down the large booth operated by GlaxoSmithKline at the conference center exposition, ringing it in a black band and waving signs reading, "Greed = Death."