A U.N. report made public yesterday warned of potential environmental harm from a salt plant that Mitsubishi and the Mexican government want to build on a lagoon in Baja California, near a U.N. World Heritage site, the biggest wildlife sanctuary in Latin America. The report is slated to be presented to the U.N. World Heritage Committee next week, but enviros, who have been anxiously awaiting its completion, have already posted it on the Internet. Enviros argue that the proposed plant would harm gray whales, which breed in the area, as well as other wildlife. A Mexican government source said no decision would be made on the saltworks project until an environmental impact study, commissioned from a group of scientists, is completed.