WASHINGTON — Health insurers will have to cover mental illness to the same degree as physical ailments as the Obama administration carries out the largest expansion of behavioral health care in a generation.

The new rules halt a practice in which insurers have been able to charge higher co-payments or deductibles for mental illness and could limit the duration of care.

In recent years, failures in treating mental illness have heightened the workload of police and pushed more emotionally disturbed people into emergency rooms. Health and Human Services Secretary Kathleen Sebelius says the new rules expand or protect behavioral health benefits for more than 60 million people.

Mental health advocates have praised the changes.

“The parity law, which also applies to policies sold through the exchanges under the Affordable Care Act, is a milestone that recognizes how integral mental health is to overall health and ends discrimination,” said David Shern, interim president and chief executive officer of Mental Health America, a lobbying group based in Alexandria, Va.

The new rules will provide the most definitive regulatory standard to date for implementing the Mental Health Parity Act of 2008. However, President George W. Bush left office without his administration ever writing rules to implement the core part of that law.

The Obama administration issued interim rules in 2010 that still left many aspects of the law open to interpretation.

In January, largely in response to the school shooting in Newtown, Conn., Obama vowed to issue final rules this year.

But the complexity of the issue, along with passage of the ACA in 2010 that included a new set of “essential health benefits” for insurance plans, delayed the final rule on mental illness, one government official said.

“Countless Americans will be safer and healthier, because these rules will enable victims of painful and debilitating mental health conditions to seek treatment before they actually commit harm to others,” commented Sen. Richard Blumenthal (D., Conn.).

But Patrick Kennedy, a former Democratic congressman from Rhode Island who was a sponsor of the mental health law, warned the rules could be meaningless unless insurers are forced to obey them.

“The rule really is only as good as the monitoring and enforcement that goes along with it,” he said.

The rules apply to almost all health plans in the nation, including new ones sold under the health care overhaul.

Although laws and regulations dating to 1996 took initial steps in requiring insurance parity for medical and mental health, “here we’re doing full parity, and we’ve also taken steps to extend it to the people covered in the Affordable Care Act,” said an official in the Obama administration. “This is kind of the final word on parity.”

According to administration officials, the rule would ensure that health plans’ co-payments, deductibles and limits on visits to health care providers are not more restrictive or less generous for mental health benefits than for medical and surgical benefits.

The regulations would clarify how parity applies to residential treatments and outpatient services.

The regulations would affect most Americans with insurance — roughly 85% of the population — whether their policies are from employer plans, other group plans or coverage purchased in the market for individual plans.

The final parity rules do not apply to health plans that manage care for millions of low-income people on Medicaid.

The administration has previously issued guidance to state health officials saying that such plans should meet the parity requirements of the 2008 law.