The US House Judiciary Committee chair has said on live TV his panel will hold former White House counsel Donald McGahn in contempt for not testifying, after the White House directed him not to.
Mr McGahn, a former White House counsel whose evidence was used extensively in the Mueller report, had been scheduled to testify in front of the committee on Tuesday, but late on Monday his lawyer said McGahn would not appear.
The US Department of Justice and White House both released statements on Monday arguing that Donald McGahn was under no obligation to give evidence.
The Democratic-led House Judiciary Committee had issued a legal order for Mr McGahn to appear on Tuesday, and its chair today told CNN Mr McGahn would be held in contempt.
"The first thing we are going to do we're going to have to hold McGahn in contempt," Jerrold Nadler said.
He said the full House would then move to enforce the contempt citation before launching court proceedings.
Mr McGahn told Mr Mueller that US President Donald Trump repeatedly tried to thwart his inquiry.
The special counsel's two-year investigation did not determine that Mr Trump conspired with alleged Russia attempts to sway the 2016 US presidential election, but listed 10 instances of possible obstruction of justice by the president.
White House press secretary Sarah Sanders said on Monday that Democrats did not like the conclusions of the Mueller report and wanted "a wasteful and unnecessary do-over".
"The former Counsel to the President cannot be forced to give such testimony, and Mr McGahn has been directed to act accordingly," her statement said, citing justice department guidance.
In its memo released on Monday, the justice department said the president's former counsel did not have to testify.
US Assistant Attorney General Steven Engel said Congress "may not constitutionally compel the President's senior advisers to testify about their official duties".
Judiciary committee chairman Jerrold Nadler said last week that he was prepared to have his panel hold a contempt vote for Mr McGahn if he ignored the subpoena issued to him last month.
"Mr McGahn is a critical witness to many of the alleged instances of obstruction of justice and other misconduct described in the Mueller report," Mr Nadler said last month.
Mr McGahn was interviewed for 30 hours by Mr Mueller's team of investigators, and was frequently cited in their 448-page report released in April.
Mr Trump later maintained he had authorised this co-operation with Mr Mueller.
The Mueller report detailed how Mr McGahn felt the president had pressured him to fire Mr Mueller and, later, write a memo saying that Mr Trump had issued no such directive.
US Attorney General William Barr was questioned this month about the matter by Senate Democrats.
He said the president had only suggested that Mr Mueller be "replaced" because of a perceived conflict of interest - and then instructed Mr McGahn to correct inaccurate media reports.
Mr McGahn left the White House in October to return to a Washington law firm, Jones Day, which represents the Trump campaign.
- BBC / Reuters