A confidential Internal Revenue Service (IRS) legal memo says tax returns must be given to Congress unless the president takes the rare step of asserting executive privilege, according to a copy of the memo obtained by The Washington Post.

The memo contradicts the Trump administration’s justification for denying lawmakers’ request for President Donald Trump’s tax returns, exposing fissures in the executive branch.

Mr Trump has refused to turn over his tax returns but has not invoked executive privilege.

Treasury Secretary Steve Mnuchin has instead denied the returns by arguing there is no legislative purpose for demanding them.

But, according to the IRS memo, which has not been previously reported, the disclosure of tax returns to the committee “is mandatory, requiring the Secretary to disclose returns, and return information, requested by the tax-writing Chairs”.

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The 10-page document says the law “does not allow the Secretary to exercise discretion in disclosing the information provided the statutory conditions are met” and directly rejects the reason that Mr Mnuchin has cited for withholding the information.

“[T]he Secretary’s obligation to disclose return and return information would not be affected by the failure of a tax writing committee ... to state a reason for the request,” it says.

It adds that the “only basis the agency’s refusal to comply with a committee’s subpoena would be the invocation of the doctrine of executive privilege”.

The memo is the first sign of potential dissent within the administration over its approach to the tax returns issue.

The IRS said the memo, titled “Congressional Access to Returns and Return Information”, was a draft document authored by a lawyer in the Office of Chief Counsel and did not represent the agency’s “official position”.

The memo is stamped “DRAFT”, it is not signed, and it doesn’t reference Mr Trump.

The agency says the memo was prepared last fall.

At the time, Democrats were making clear they would likely seek copies of President Trump’s tax returns under a 1924 law that states that the IRS “shall” turn over tax returns to Congress.

Precisely who wrote the memo and reviewed it could not be learned.

The agency says IRS Commissioner Charles Rettig and current chief counsel Michael Desmond, who took over in February, were not familiar with it until a Washington Post inquiry this week. The IRS says it was never forwarded to Treasury.

Executive privilege is generally defined as the president’s ability to deny requests for information about internal administration talks and deliberations.

On Friday, Mr Mnuchin rejected a subpoena from the House Ways and Means Committee to turn over the tax returns, a move that will now likely lead to a court battle.

Mr Mnuchin has criticised the demands as harassment that could be directed against any political enemy, arguing Congress lacks a “legitimate legislative purpose” in seeking the documents.

Breaking with precedent, Mr Trump has refused to provide tax returns, saying without evidence they are under audit.

Mr Mnuchin and other senior staff members never reviewed the IRS memo, according to a Treasury spokesman.

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But the spokesman said it did not undermine the department’s argument that handing over the president’s tax returns would run afoul of the Constitution’s mandate that information given to Congress must pertain to legislative issues.

The spokesman said the secretary is following a legal analysis from the Justice Department that he “may not produce the requested private tax return information”.

Both agencies have denied requests for copies of the Justice Department’s advice to Treasury.

Some legal experts said the memo provides further evidence that the Trump administration is using shaky legal foundations to withhold the tax returns.

“The memo is clear in its interpretation of the law that the IRS shall furnish this information,” said William Lowrance, who served for about two decades as an attorney in the IRS chief counsel’s office and reviewed the memo at the request of The Washington Post.

Daniel Hemel, a professor at the University of Chicago Law School who also reviewed the memo, said the document suggests a split over Mr Trump’s returns between career staffers at the IRS and political appointees at that agency and the Treasury Department.

“The memo writer’s interpretation is that the IRS has no wiggle room on this,” Mr Hemel said. “Mnuchin is saying the House Ways and Means Committee has not asserted a legitimate legislative purpose. The memo says they don’t have to assert a legitimate legislative purpose – or any purpose at all.”

The administration has resisted a range of House inquiries, although a federal judge on Monday ruled the president’s accounting firm must turn over his financial records to Congress.

Treasury Department officials said there had been extensive discussions about the tax return issue, with one official saying the issue put the agency in a difficult spot because President Trump has pre-determined the outcome – and Mr Mnuchin is a Trump ally who was the finance chair of his 2016 campaign.

“The decision has been made,” this official said, speaking on the condition of anonymity to discuss private conversations. “Now it’s up to us to try to justify it.”

Mr Trump has told advisers he will battle the issue to the Supreme Court, according to people familiar with the matter.

President Trump recently has argued that the tax returns were an issue in the 2016 election, but he won anyway – so they should no longer be an issue.

Last week, Mr Mnuchin told a Senate panel that Treasury Department lawyers held an early discussion about disclosing the tax returns long before Democrats officially demanded the documents in April.

He did not reveal details of that deliberation or say what, if any, legal memos he had reviewed.

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Some legal experts have held that the law is clear in giving Congress the power to compel the provision of the returns.

But other former government lawyers, including two who served in the Ronald Reagan and George HW Bush administrations, have argued that the law is unconstitutional and could lead to widespread abuses of taxpayer privacy for political aims.

The IRS memo describes how and why Congress has the authority to access tax returns, explaining the origin of the provision and how it has been interpreted over the decades.

It highlights the special powers given to three committees for compelling tax returns: the House Ways and Means Committee, the Senate Finance Committee and the Joint Committee on Taxation.

Other congressional committees, the memo emphasises, do not have the same authority to compel returns.

When it comes to the Ways and Means Committee, the obligation to divulge the returns “would not be affected by the failure” to give a reason for the request.

By contrast, other committees “must include a purpose for their request for returns and return information when seeking access”, the memo states.

“One potential basis” for refusing the returns, the memo states, would be if the administration invoked the doctrine of executive privilege.

But the IRS memo notes that executive privilege is most often invoked to protect information, such as opinions and recommendations, submitted as part of formulating policies and decisions.

It even says the law “might be read to preclude a claim of executive privilege”, meaning the law could be interpreted as saying executive privilege cannot be invoked to deny a subpoena.

Earlier this month, the non-partisan Congressional Research Service (CRS) published a review of Section 6103 that found the code “evinces no substantive limitations” on the Ways and Means Committee’s authority to receive the tax returns.

But, the CRS report added, the committee’s authority “arguably is subject to the same legal limitations that generally attach to Congress’ use of other compulsory investigative tools”, including the need to serve some “legislative purpose” and not breach constitutional rights.