U.S. consumers have been forced to pay billions of dollars in medical bills health insurers should have covered, a Senate committee said Wednesday.

That's because private insurers have systematically underpaid for out-of-network care, the Senate Commerce Committee said in its report.

Insurers normally pay a percentage of the "usual-and-customary" rates, but the process remains a mystery to most consumers and is difficult to challenge, The Washington Post reported.

The report says insurers typically relied on numbers from Ingenix Inc., a wholly owned subsidiary of health insurer UnitedHealth Group, to determine "usual-and-customary" rates for out-of-network care, the Post said. Ingenix determined benchmarks that kept usual-and-customary rates low, placing more of the cost burden on consumers, the report said.

The report also revealed the Ingenix data came from the insurers that bought the company's information and that insurers often removed high charges from their data, the Post said.

In March testimony to the Commerce Committee, UnitedHealth Group's chief executive, Stephen J. Hemsley, said he regretted the conflict of interest between his company and Ingenix, the report said.

But Hemsley said he stands by the integrity of the Ingenix data and the way the insurer used the data, the Post reported.