Under the provision to set up state-based marketplaces, subsidies are supposed to be available to many lower- and middle-income people who do not have access to coverage from employers or other sources. It may be difficult, however, for officials running the exchanges to know who is entitled to subsidies if employers do not report information on the coverage they provide to workers.

Enrollment in the exchanges is to begin Oct. 1, with insurance coverage taking effect on Jan. 1. “We are on target to open the health insurance marketplace on Oct. 1 where small businesses and ordinary Americans will be able to go to one place to learn about their coverage options and make side-by-side comparisons of each plan’s price and benefits before they make their decision,” Valerie Jarrett, Mr. Obama’s senior adviser and liaison to the business community, wrote on the White House Web site.

But even some supporters of the law dispute that the establishment of the health insurance exchanges is on schedule, especially since progress varies by state and some Republican-led states are resisting the health care law and withholding resources for putting it into effect.

Much of the administration’s public effort, especially at the Department of Health and Human Services, has been directed toward spreading the word to uninsured Americans, especially younger and healthy individuals whose participation is needed to help keep down premiums for everyone else. About 85 percent of Americans are insured, so most individuals will be unaffected, at least initially.

Behind the scenes, however, the administration has been fielding questions and criticisms from businesses about the reporting requirements — especially the Treasury Department, which has responsibility, given its oversight of the tax reporting system.

Employer groups were quick to applaud the delay. At the U.S. Chamber of Commerce, which has strongly opposed the law, Randy Johnson, senior vice president for labor, immigration and employee benefits, said in a statement, “The administration has finally recognized the obvious — employers need more time and clarification of the rules of the road before implementing the employer mandate.”

E. Neil Trautwein, a vice president of the National Retail Federation, said the delay “will provide employers and businesses more time to update their health care coverage without threat of arbitrary punishment.”