BILLINGS, Mont. • Congressional investigators have found problems with federal coal sales that might have cost taxpayers $200 million or more in lost revenue, a senator said Tuesday.

Citing a new report by investigators at the nonpartisan U.S. Government Accountability Office, Sen. Edward Markey, D-Mass., called for the sales to be suspended until the problems are rectified.

More than 40 percent of U.S. coal production — or about 450 million tons a year — comes from public lands leased by the government to mining companies under the century-old Mineral Leasing Act.

Those leases bring in more than $1 billion in annual revenue.

While exports of the fuel to lucrative Asian markets have surged in recent years, the rules for leasing government-owned coal have remained largely unchanged since 1990.

That’s stirred concerns that companies could be shortchanging taxpayers by buying coal cheaply from the government based on U.S. market prices and then selling it at a premium overseas.

A sweeping, 19-month examination of the Interior Department’s coal-leasing program by the GAO revealed widespread inconsistencies in how the government values public coal reserves that are leased to private mining companies.