In a letter to House Ways and Means Committee Chairman Richard Neal, D-Mass., Mnuchin said that after conferring with the Justice Department, he has determined that the request "lacks a legitimate legislative purpose" and that because of that the request would be denied.

The formal denial, coming weeks after acting White House chief of staff Mick Mulvaney said Congress will never get those tax returns, sets the stage for yet another fight over documents sought by the Democratic-led House of Representatives from the Republican Trump's administration.

Treasury Secretary Steven Mnuchin on Monday said he will not allow President Donald Trump 's tax returns to be released to Congress by the IRS, as a powerful oversight committee has requested.

"I am informing you now that that [Treasury] Department may not lawfully fulfill the Committee's request," Mnuchin wrote to Neal, whose committee is one of three congressional panels with the power to request a president's income tax returns.

House Democrats had asked the IRS to release six years of Trump's personal and business tax returns. Experts have said the U.S. tax code mandates that anyone's tax returns "shall" be released to one of the authorized panels if they request them.

Mnuchin's letter called that request "unprecedented" and said it also "presents serious constitutional questions, the resolution of which may have lasting consequences for all taxpayers."

Mnuchin also said that the Justice Department intends to "memorialize its advice in a published legal opinion as soon as practicable."

Neal, in a statement, said, "Today, Secretary Mnuchin notified me that the IRS will not provide the documents I requested under Section 6103 of the Internal Revenue Code. I will consult with counsel and determine the appropriate response."

Trump has repeatedly rebuffed requests to release his tax returns to the public, saying they are being audited. But Trump's former personal attorney Michael Cohen has testified to Congress that he has seen no proof that Trump was being audited.

Cohen entered a federal prison in upstate New York on Monday to begin serving a three-year sentence for multiple crimes.

In April, Trump said Americans don't care if they can see his tax returns.

"Remember, I got elected last time," Trump told reporters then. "The same exact issue, with the same intensity, which wasn't very much. Because frankly, the people don't care."

However, a poll conducted around the same time found that 51% of voters supported Democrats' bid to obtain his tax returns, compared with 36% of voters who oppose that effort.