Donald Trump's tax returns may finally become public after MSNBC journalist Rachel Maddow said her network would release them.

The Republican refused to release his tax returns during his presidential campaign, in an unusual move that prompted criticism from Democrats and members of the GOP alike.

But Maddow wrote on Twitter on 14 March: "BREAKING: We've got Trump tax returns. Tonight, 9pm ET. MSNBC. (Seriously)."

The journalist did not explain how the channel had managed to obtain the returns, nor which dates they would cover.

However, the promise of the long-awaited details of the president's tax returns is sure to draw a big audience.

Republicans Mark Sanford and Walter Jones had put their names to a letter requesting the Senate Finance Committee and the House Ways and Means Committee demand the president's tax documents.

The letter, seen by The Hill, reads: "Disclosure would serve the public interest of clarifying President Trump's conflicts of interest in office, the potential for him to personally benefit from tax reform, and ensure that he is not receiving any preferential treatment from the IRS."

BREAKING: We've got Trump tax returns. Tonight, 9pm ET. MSNBC. (Seriously). — Rachel Maddow MSNBC (@maddow) March 14, 2017

Trump's failure to turn over his tax returns during his presidential campaign was unusual and controversial. He claimed at the time that he could not release the accounts because he was being audited.

The president also made the unusual response to allegations that he had not paid federal taxes for 18 years made him "smart".