Donald Trump’s tax returns must be handed to House Democrats by 23 April, a leading committee chair said on Saturday.

Democrats initially set a 10 April deadline for the returns but this week treasury secretary Steven Mnuchin said he needed more time to assess issues raised by the request.

On Saturday the chairman of the House ways and means committee, Richard Neal of Massachusetts, wrote to Internal Revenue Service commissioner Charles Rettig to say a failure to comply with the new deadline would be “interpreted as a denial of my request”.

Constitutionally, Neal has the power to demand the IRS release tax returns for any US individual. He has asked for six years of the president’s personal and business returns.

In his letter, he wrote that his power to make the demand “is unambiguous and raises no complicated legal issues”.

Trump was at his golf course in Virginia on Saturday but the White House has already said it will refuse to release such information for a president who as a candidate broke with convention but not law by refusing to make his tax returns public.

Last week, acting chief of staff Mick Mulvaney told Fox News Sunday Democrats would “never” see Trump’s tax returns. Claiming the request was purely political, Mulvaney added: “That is not going to happen and they know it.”

Trump’s personal attorney, William Consovoy, has called the request a “gross abuse of power”. Mnuchin used similar language in a letter to Neal this week.

In his letter to Rettig, a Trump appointee, Neal said concerns expressed by the administration “lack merit” and added: “Judicial precedent commands that none of the concerns raised can legitimately be used to deny the committee’s request.

“It is not the proper function of the IRS, treasury or justice [departments] to question or second guess the motivations of the committee or its reasonable determinations regarding its need for the requested tax returns and return information.”

Legal experts expect a final denial by the Trump administration to lead to a subpoena from House Democrats and a fight in the courts.

Trump has repeatedly claimed to be unable to release his tax returns because he is under audit. Experts have repeatedly pointed out that being under audit does not preclude the release of such information.

The president’s tax returns have duly become a Holy Grail for his opponents and a source of constant speculation, not least during the Mueller investigation into links between the Trump campaign and Russia.

Trump on tax, in September 2016.

In September 2016, in his first presidential debate against Hillary Clinton, Trump said not paying federal taxes “makes me smart”.

In October that year, shortly before the election, the New York Times reported that Trump may not have paid federal income tax in 18 years.

In March 2017, the MSNBC host Rachel Maddow obtained and made public a portion of Trump’s tax return for 2005, which showed he had paid $35m in federal taxes that year. It was thought Trump himself might have been behind the leak.

In October 2018, the New York Times released a major investigative report which said the Trump family engaged in “dubious tax schemes during the 1990s, including instances of outright fraud”.

A lawyer for Trump said the Times report contained “allegations of fraud and tax evasion [that] are 100% false, and highly defamatory” and said “there was no fraud or tax evasion by anyone”. An official inquiry followed.

In November 2018, shortly after Democrats took back the House, putting them in position to demand the relevant information, Trump said at a press conference his tax returns were too complicated for the public to understand.