WASHINGTON — The Internal Revenue Service has no choice but to honor congressional requests for President Trump’s tax returns unless he invokes executive privilege to protect them, according to a draft legal memo written by agency staff members.

The memo appears to undercut the reasoning offered by Treasury Secretary Steven Mnuchin, who has refused to comply with Democrats’ requests because they lack a “legitimate legislative purpose.” Mr. Mnuchin said he made his decision after consulting with lawyers from the Treasury Department, the I.R.S. and the Justice Department.

But the I.R.S. memo says the Treasury secretary does not have the authority to deny tax-writing committees’ requests for taxpayer returns.

The House Ways and Means Committee is seeking six years of the president’s personal and business tax returns. Mr. Trump has not asserted executive privilege to block the request, and his personal lawyers have asked the Treasury Department not to release them. The president has indicated that he is prepared to take the legal fight over his returns to the Supreme Court.