The New York Times says it has printouts from the future president's official IRS tax transcripts, including figures from his federal tax form.
It said Trump reported business losses of $US46.1 million in 1985, and a total of $US1.17 billion in losses for the 10-year period.
After comparing Trump's information with that of other "high-income earners," the Times concluded that Trump "appears to have lost more money than nearly any other individual American taxpayer."
Because of his business losses, the newspaper reported, Trump did not pay income taxes for eight of the 10 years.
Treasury Secretary Steven Mnuchin on Monday refused to do so, saying the panel's request "lacks a legitimate legislative purpose."
Mnuchin's move, which had been expected, is likely to set a legal battle into motion.
The chief options available to Democrats are to subpoena the IRS for the returns or to file a lawsuit.
Trump is the first president since Watergate to decline to make his tax returns public.