Taxpayers who received unemployment benefits last year — but who filed their federal tax returns before a new tax break became available — could receive an automatic refund as early as May, the Internal Revenue Service said on Wednesday.

The latest pandemic relief legislation — signed into law on March 11, in the thick of tax season — made the first $10,200 of unemployment benefits tax-free in 2020 for people with modified adjusted incomes of less than $150,000. (Married taxpayers filing jointly can exclude up to $20,400.)

But some Americans had already filed their tax returns by March and have been waiting for official agency guidance. Millions of U.S. workers filed for unemployment last year, but the I.R.S. said it was still determining how many workers affected by the tax change had already filed their tax returns.

On Wednesday, the I.R.S. confirmed that it would automatically recalculate the correct amount of benefits subject to taxation — and any overpayment will be refunded or applied to any other outstanding taxes owed. The first refunds are expected to be issued in May and will continue into the summer.