WASHINGTON — In a major surprise on the new health-care law, the Obama administration said Friday it would not define a single uniform set of "essential health benefits" that must be provided by insurers for tens of millions of Americans. Instead, each state will be allowed to specify benefits within broad categories.

The move would allow significant variations in benefits from state to state, much like the current differences in state Medicaid programs and the Children's Health Insurance Program.

The announcement Friday sparked criticism from interest groups on all sides of the issue. Consumer advocates worried that millions of Americans could end up with insurance substantially less comprehensive than the law's drafters intended.

Representatives of employers and insurers warned of an opposite scenario: A state could make the benefit package so comprehensive that resulting health plans would be prohibitively expensive.

Obama officials have yet to spell out key details, but Friday's announcement offered a road map for how the administration intends to implement the health-care law's "essential benefits" mandate.

Under the mandate, the benefits package must include such fundamentals as inpatient and outpatient care, emergency services, maternity and childhood care, prescription drugs, preventive screenings and labs.

It also must cover mental-health and substance-abuse treatment, as well as rehabilitation for physical and cognitive disorders, and dental and vision care for children. Such additional benefits often are not covered fully by frugal plans that are the best many small businesses can afford.

The mandate will apply to new policies purchased by individuals or small businesses beginning in 2014.

Kathleen Sebelius, secretary of Health and Human Services, had been expected to provide details about what services and benefits must be provided in each of 10 coverage areas, for instance, maternity care. Instead, she said the federal government would respect the states' role, giving them "the flexibility to design coverage options that meet their unique needs."

"The proposal we're putting forward today reflects our commitment to giving states the flexibility they need," Sebelius said. It's a prickly relationship, with 26 states, including Washington, asking the Supreme Court to toss out the law.

Under the guidance issued Friday, state leaders can define their benefits by using an existing major health plan in their state as a "benchmark."

That means some states may require insurers to cover services such as chiropractic therapy or in vitro fertilization, while others may not. The rules will not affect co-pays, cost-sharing and deductibles, which play a major role in determining premiums.

If a benchmark plan does not cover services in the 10 mandated categories, states will have to come up with supplementary requirements through an as-yet-unspecified process.

Sebelius said the arrangement would ensure that state leaders can "tailor" health-insurance requirements to local conditions and priorities. "Coverage that works in Florida may not work in Nebraska," she said.

She also stressed that all 30 million Americans in plans affected by the rule would be guaranteed a minimum level of coverage that many currently lack. According to government estimates, more than 1 million Americans will gain prescription-drug coverage, for instance, and more than 8 million will gain coverage for maternity care.

Consumer advocates expressed concern that, unless the services covered under the general rubric of each category are specified, many plans will be able to comply with the mandate while offering substandard care.

"Let's say a plan only covers generic drugs. Well, a lot of cancer drugs are only brand name," said Stephen Finan, senior director of policy at the American Cancer Society Cancer Action Network.

Also potentially subject to abuse, consumer advocates said, is a provision that would allow insurers to modify benefits they offer under the minimum package as long as the total cost is not changed.

Employers and insurers also were unsatisfied. Many states require coverage of services beyond those in the minimum package, said Neil Trautwein, an official at the National Retail Federation who chairs a group of employers and health plans called the Essential Health Benefits Coalition. Friday's proposal included a provision that, for the first two years, effectively would allow states to include those additional requirements as part of the minimum-benefits package, potentially increasing the cost of plans.

Compiled from The Washington Post, the Tribune Washington bureau, The New York Times and The Associated Press