A multibillion-dollar gap between what public companies book as expenses for their executives’ stock options and what they report to the IRS under two sets of rules is costing the Treasury billions in lost revenue, a key senator seeking to end the discrepancy said Tuesday.

Companies are reporting deductions for stock options to the Internal Revenue Service that far exceed what they are reckoning and disclosing to shareholders as expenses against their bottom line, Sen. Carl Levin, D-Mich., said at a hearing of the Senate Homeland Security and Governmental Affairs investigative subcommittee.