U.S. healthcare reforms have enabled 2.5 million young adults to obtain insurance coverage, the U.S. government said on Wednesday, up from 1 million earlier this year.

Federal officials credited the gains to the Affordable Care Act, legislation championed by President Barack Obama that took effect last year and is deemed the biggest overhaul of the U.S. healthcare system in nearly 50 years.

The law aims broadly to eventually provide medical coverage to more than 30 million uninsured Americans, and already allows young adults to stay on their parents' private insurance plans through age 26.

Since the policy helping young adults took effect in September 2010, the percentage of adults ages 19 to 25 covered by a private health insurance plan has increased significantly, the U.S. Department of Health and Human Services said.

Federal officials said data from the first three months of 2011 showed that 1 million more young adults had coverage compared with a year ago.

The rise to 2.5 million reported on Wednesday was due to graduation from high school and college in May and June of students who otherwise would have lost coverage, they said.

"Moms and dads around the country can breathe a little easier knowing their children are covered," Kathleen Sebelius, Secretary of Health and Human Services, said in a release.

The agency said trends seen through early 2011 are consistent with those reported previously by the Census Bureau, a Gallup survey and the National Health Interview Survey conducted by a branch of the U.S. Centers for Disease Control and Prevention.

Dozens of states and a field of contenders for the Republican presidential nomination have attacked the Affordable Care Act, sometimes dubbed "Obamacare." They hope to repeal the legislation, saying it is a symbol of intrusive government seeking to raise taxes and burden businesses with new regulation.

The U.S. Supreme Court is preparing to review the reforms, as a sharply divided public watches from the sidelines. (Reporting By Ransdell Pierson in New York, editing by Matthew Lewis)