Washington (AFP) - Republican presumptive nominee Donald Trump's refusal to release his tax returns until after the November election should disqualify him as a presidential candidate, the party's 2012 standardbearer Mitt Romney warned Wednesday.

"It is disqualifying for a modern-day presidential nominee to refuse to release tax returns to the voters, especially one who has not been subject to public scrutiny in either military or public service," Romney wrote in a Facebook post.

Romney, himself a multi-millionaire, said the only logical explanation for Trump's failure to show his back taxes was that they contain "a bombshell of unusual size."

"Mr. Trump, tear down that tax wall," Romney said.

Trump is a billionaire New York real estate mogul whose financial dealings have been under intense scrutiny for years.

He said in an interview with the Associated Press published Wednesday that despite pressure to release his returns, he likely will not do so before the November election due to an ongoing financial audit.

"There's nothing to learn from them," Trump said in the interview, adding that he did not believe they were of interest to voters.

Later Wednesday, with pressure snowballing, Trump tweeted that he did not refuse to release his returns before the election.

"In interview I told @AP that my taxes are under routine audit and I would release my tax returns when audit is complete, not after election!" he wrote.

Failure to reduce his tax returns would mark a clear break with political tradition.

Every major presidential and vice presidential nominee since 1976 has released his or her returns, according to fact-checking website PolitiFact.

Romney's father George Romney famously released 12 years of tax returns prior to the 1968 election, when he ran for president.

Hillary Clinton, the likely Democratic presidential nominee, has released her tax returns dating back to 1977.