NEW YORK (Reuters) - U.S. Northeast power companies likely will not race to buy permits to emit the main greenhouse gas in the country's first carbon auction later this month because the region's emissions of the gas have slipped over the last few years, experts said.

The Regional Greenhouse Gas Initiative, a group of 10 states in the U.S. Northeast that formed the first U.S. greenhouse market, will regulate carbon dioxide emissions from power plants starting next year. It will hold its first auction for permits to pollute more than 12 million tons of emissions on September 25.

But bids in the auction may be soft because CO2 emissions in 2007 were about 9 percent below the cap the group set on emissions of 188 million tons of carbon dioxide, according to Environment Northeast, a green group that helped form the market.