General Motors Corp. (GM) will send letters to at least 1,000 auto dealers by Friday warning that their businesses are considered underperforming.

The auto maker is trying to cull 2,600 dealers by 2010 as part of a downsizing ordered by the Obama administration as the company struggles to avoid a bankruptcy filing.

The company recently completed an evaluation of its dealer network that determined which dealerships should go and which should stay, GM spokeswoman Susan Garontakos said Wednesday. The development was first reported by Automotive News.

The reductions are part of sweeping moves by GM as it tries to shrink itself in line with sharply lower sales levels in the U.S. GM's dealer network is sized for a time when the auto maker commanded a much greater share of the U.S. market and faced fewer competitors. The company has said it needs smaller, healthier dealers to be competitive. But dealers, protected by state franchise laws, have fought GM's efforts to shrink its network.

Chrysler LLC, in bankruptcy protection, is working on similar reductions.

-By Sharon Terlep, Dow Jones Newswires; 248-204-5532; sharon.terlep@dowjones.com