Speaking at a White House forum on the law and women's health issues, Sebelius said the administration remains "confident and optimistic" that the 2010 Patient Protection and Affordable Care Act will be upheld as constitutional.

But if the ruling proves unfavorable, she added: "We'll be ready for court contingencies." Sebelius offered no details about how the administration would respond.

The reform law, President Barack Obama's signature domestic achievement, would extend coverage to more than 30 million uninsured people by expanding the Medicaid program for the poor and establishing regulated state insurance markets where lower income people could buy subsidized plans.

The court could overturn the entire reform package or eliminate selected provisions including the individual mandate, which requires most Americans to obtain health coverage by the time the law comes into full effect in 2014.

If the court strikes down the mandate, considered a pivotal issue in the case, that could cripple the law's ability to keep insurance costs down by compelling younger, healthier people to buy coverage.

Most alternative ways to bring down insurance costs would require cooperation from Congress, an uncertain prospect as many Republicans have vowed to try to repeal the law.

Sebelius warned that the court striking down the entire law would have a "pretty cataclysmic impact" on millions of people, including 60,000 with pre-existing conditions who have been able to gain health coverage through a program created by the law.

The government would have to establish new rates for Medicare, the popular healthcare program for the elderly and disabled, because higher payment rates for physicians who provide preventive and primary care would cease to exist.

Millions of young adults would no longer be able to remain on their parents' health plans until age 26, and a host of preventive procedures including mammograms and childhood immunizations would no longer be available without deductible payments or co-pays, she said.

"We assume that if the court would strike that down, those all would cease to be the law of the land," Sebelius said.

"What we're doing right now, quite frankly, is just working as hard as we possibly can to get ready for 2014," she added. "We think it's the best preparation to anticipate that the law is fully constitutional."

(Reporting By David Morgan; Editing by Vicki Allen)

• A Permanent Economic Emergency