A federal judge in Washington has ruled against U.S. President Donald Trump in a financial records dispute with Congress.

Judge Amit Mehta's ruling says Trump cannot block the House subpoena of his financial records.

The decision is a setback for the Trump administration amid a widespread effort by the White House and the president's lawyers to refuse to co-operate with congressional requests for information and records.

Trump and his business organization had sued to block the subpoena issued in April to Mazars LLP, an accounting firm for the president and Trump Organization.

Mehta also denied a request by Trump to stay his decision pending an appeal.

Mazars, an international company that specializes in audit, accounting, tax and advisory services has said it will 'comply with all legal obligations.' (Charles Platiau/Reuters)

Last Tuesday, Mehta heard oral arguments on whether Mazars must comply with a House of Representatives oversight committee subpoena.

Mehta said at the time that financial records from Trump's long-time accounting firm would be part of a "proper subject of investigation" by Congress, appearing to side with Democratic lawmakers seeking more oversight of the president.

In his ruling Monday, Mehta wrote that the committee "has shown that it is not engaged in a pure fishing expedition for the president's financial records" and that the Mazars documents might assist Congress in passing laws and performing other core functions.

"It is simply not fathomable that a Constitution that grants Congress the power to remove a president for reasons including criminal behaviour would deny Congress the power to investigate him for unlawful conduct — past or present — even without formally opening an impeachment inquiry," Mehta said.

1 week to comply

Mehta said Mazars has seven days to comply with the subpoena. Trump's lawyers are almost certain to urge an appeals court to extend the deadline and then reverse Mehta's decision.

It was the first time a federal court had waded into the tussle about how far Congress can go in probing Trump and his business affairs.

Speaking to reporters in Washington after the ruling, Trump called the decision "crazy" and said it would be appealed.

"It's totally the wrong decision by obviously an Obama-appointed judge," Trump said.

Trump is refusing to co-operate with a series of investigations into issues ranging from his tax returns and policy decisions to his Washington hotel and his children's security clearances.

Trump's lawyers argued that Congress is on a quest to "turn up something that Democrats can use as a political tool against the president now and in the 2020 election."

The House oversight committee claims sweeping investigative powers and says it needs Trump's financial records to examine whether he has conflicts of interest or broke the law by not disentangling himself from his business holdings as previous presidents did.

Lawyers for Trump and the Trump Organization, his company, last month filed a lawsuit to block the committee's subpoena, saying it exceeded Congress's constitutional limits.

Mazars has avoided taking sides in the dispute and said it will "comply with all legal obligations."