The US treasury department will not comply with a deadline set by Democratic legislators to hand over Donald Trump’s tax returns, the secretary of the treasury Steve Mnuchin announced.

“The committee requests the materials by April 10, but the treasury department will not complete its review of your request by that date,” Mnuchin wrote in a letter to Ways and Means Committee chairman Richard Neal.

In his letter, Mnuchin cited concerns over “an abuse of authority” and wrote that the law cited in Neal’s request could not be used “for purposes of embarrassing or attacking political figures of another party”.

Emphasizing the political weight of the request, he added that the department is seeking counsel from the department of justice “to ensure that our response is fully consistent with the law and the Constitution”.

Trump has consistently refused requests by politicians, journalists and others, stating his reason for not doing so is that the returns are under audit. Tax and legal experts have said, however, that an audit should not prevent their public release, a practice presidents have followed for decades.

Democrats on the House of Representatives’ tax committee want to review Trump’s returns chiefly as part of their investigations into possible conflicts of interest posed by his continued ownership of extensive business interests even as he serves the public as president.

Republicans oppose release of the returns, arguing that it would politicize tax data.

Mnuchin’s letter was supported by congressman Kevin Brady, the top Republican on the Ways and Means Committee, who referred to his committee’s request for the returns as “political fishing expeditions”.

“The treasury department is right to carefully review the privacy impact this request would have on every taxpayer,” he added in a statement.

The returns of US taxpayers are generally held as confidential by the Internal Revenue Service (IRS), which is part of the treasury department.

“There is a high likelihood this ends up in the courts, which is, in a way, unusual because typically when there’s a request like this by Congress, the matter is settled more politically with a compromise,” said Jessica Levinson, a professor at Loyola Law School in Los Angeles who specializes in political process law. “But in this case both sides have really dug in.”

Legal experts have said that little case law would be available to guide judges if Congress were to take the administration to court over the issue.

Asked about the matter on Wednesday, Trump once again said his tax returns were being audited by the IRS. “I would love to give them, but I’m not going to do it while I’m under audit. It’s very simple,” Trump told reporters.

House tax committee chairman Richard Neal last week requested six years of Trump’s personal and business returns from the IRS, invoking a seldom-used law entitling him to make such a request.

Mnuchin said in an interview on CNBC television on Wednesday that it was appropriate for his agency to consult with the White House counsel on Neal’s request. Mnuchin had said on Tuesday that treasury department lawyers had held “informational” discussions with the White House about an expected request for the returns, a step that Democrats criticized as uncalled-for under the law.

Democrats say a 1924 statute requires the treasury secretary to turn over tax returns to the chairmen of the congressional tax committees who ask for them for investigative purposes.