The US Justice Department says the Treasury Department must provide the House Ways and Means Committee former President Donald Trump’s tax returns, apparently ending a long legal showdown over the records.

During the Trump administration, then Treasury Secretary Steven Mnuchin said he wouldn’t turn over the tax returns because he concluded they were being sought by Democrats, who control the House of Representatives, for partisan reasons.

The committee sued for the records under a federal law that says the Internal Revenue Service “shall furnish” the returns of any taxpayer to a handful of top lawmakers.

The committee said it needed Mr Trump’s taxes for an investigation into whether he complied with tax law.

Manhattan District Attorney Cyrus Vance Jr already has obtained copies of Mr Trump’s personal and business tax records as part of an ongoing criminal investigation.

Mr Trump tried to prevent his accountants from handing over the documents, taking the issue to the Supreme Court. The justices rejected Mr Trump’s argument that he had broad immunity as president.

A House committee sued the Trump administration in federal court on Tuesday for access to his tax returns, setting up a legal showdown over the records.

The lawsuit is the culmination of a long-running fight between Democrats and Mr Trump over the returns, dating back to the 2016 campaign, when Mr Trump claimed that he could not release them due to an IRS audit.

The records hold the promise of information that Mr Trump has guarded from public view, including about his business entanglements, relationships with foreign creditors and governments, and the value of his assets.

The committee originally demanded six years of Trump’s tax records in early April under a law that says the Internal Revenue Service “shall furnish” the returns of any taxpayer to a handful of top lawmakers.

But Treasury Secretary Steven Mnuchin told the committee in May that he wouldn’t be turning over the returns to the Democratic-controlled House.

Mr Mnuchin concluded that the Treasury Department is “not authorised to disclose the requested returns and return information”. 

The Justice Department, in a legal opinion, backed Mr Mnuchin’s position, saying the request lacked a legitimate legislative purpose and was an “unprecedented” use of congressional authority.

The committee said it’s not required to explain to the Treasury Department its reasons for seeking the information but that in this case the committee’s need is “evident”.

“Without reviewing the requested return materials, the committee cannot ensure that the IRS’s audit process is functioning fairly and effectively," the lawsuit said.

The president has “declined to follow the practice of every elected President since Richard Nixon of voluntarily disclosing their tax returns,” the lawsuit said.

It’s unclear how long it will take to resolve the lawsuit. Fights between Congress and the executive branch can in some cases take years, and the administration may be eager to make the lawsuit last in order to delay providing records. But if, as the committee suggests, the fight comes down to the text of what the law requires, a resolution could come more quickly, though it could be appealed.

Steve Rosenthal, a senior fellow at the nonpartisan Urban-Brookings Tax Policy Centre, said the committee is on strong legal footing with the lawsuit because “it is entitled to oversee and investigate the executive branch.”

The chairman of the Ways and Means Committee, Democrat Richard Neal, said in a statement that despite the Treasury Department’s “mandatory obligation”, it had “failed to comply with the law”.

Republican Senator Chuck Grassley, the chairman of the Senate Finance Committee said: “It should be alarming to every American that there are attempts by elected officials to weaponise the IRS for partisan political purposes,” Grassley said earlier this year.