The Obama administration has been making billions of dollars in payments to insurance companies under the health law without being able to confirm just how much it owes each insurer, according to an inspector general’s report to be released Tuesday.

The federal government has paid subsidies for many enrollees’ premiums and deductibles directly to insurers since January 2014, when key health-law changes kicked in and millions of lower-income Americans started gaining coverage from plans they picked through HealthCare.gov or state equivalents.