Washington District Judge Amit Mehta also denied a request by Trump to stay his decision pending an appeal.

Last Tuesday Mehta heard oral arguments on whether the president's accounting firm Mazars LLP must comply with a House of Representatives Oversight Committee subpoena.

Mehta said in Monday's ruling the committee "has shown that it is not engaged in a pure fishing expedition" and the documents might assist Congress pass laws and perform other core functions.

"It is simply not fathomable 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.

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.

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

The stand-off deepened on Monday when Trump told former White House counsel Don McGahn to defy a subpoena to testify about Special Counsel Robert Mueller's Russia investigation before a different congressional committee.

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

The House Oversight Committee claims sweeping investigative power 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.

Australian Associated Press