Treasury Secretary Steve Mnuchin told Congressional Democrats that he would not be honoring their request to provide six years of President Donald Trump's taxes, likely setting up a legal battle.

Mnuchin rejected the demand in a letter to Congress released Monday.

"In reliance on the advice of the Department of Justice, I have determined that the Committee's request lacks a legitimate legislative purpose," Mnuchin wrote, "the Department is therefore not authorized to disclose the requested returns and return information."

The Democratically controlled Ways and Means Committee had set a deadline for compliance in April, but Mnuchin responded by saying that he needed to seek guidance from the Department of Justice.

Rep. Richard Neal (D-Mass.) responded with a curt statement.

"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," he said.

The refusal will likely lead to a legal battle over the taxes that the president has said he would reveal after his audit was over.

According to a Quinnipiac University poll, 29 percent of Americans polled said he shouldn't release his tax returns, while 64 percent said he should.

Here's the latest on the Mnuchin refusal: