The Justice Department sent Mueller a letter on Monday night saying it expects him to limit his testimony to what is already publicly available in the report on the investigation into alleged Russian meddling and collusion with President Donald Trump's campaign.
The letter prompted Democratic lawmakers to hit back at US Attorney General Barr for trying to prevent information damaging to Trump from being revealed.
Mueller’s testimony before the House Judiciary Committee is scheduled to begin at 8:30 a.m. local time (12:30 GMT) and will last for three hours. After a brief recess, Mueller is scheduled to testify before the House Intelligence Committee at noon.
The chairmen of the two committees hope Mueller will disclose more substantial information than what was in the report released by the special counsel's office in April.
"All members - Democrats and Republicans - of both committees will have a meaningful opportunity to question the Special Counsel in public and the American people will finally have an opportunity to hear directly from Mr. Mueller about what his investigation has uncovered", committee chairmen Jerry Nadler and Adam Schiff said in a joint statement earlier this month.
On Monday, President Donald Trump criticised Mueller for agreeing to testify in Congress after concluding the probe, saying the testimony will not yield any new information.
Highly conflicted Robert Mueller should not be given another bite at the apple. In the end it will be bad for him and the phony Democrats in Congress who have done nothing but waste time on this ridiculous Witch Hunt. Result of the Mueller Report, NO COLLUSION, NO OBSTRUCTION!...— Donald J. Trump (@realDonaldTrump) 22 июля 2019 г.
Mueller said he intends to confine his testimony to the statements he made in the final report on the Russia probe.
During an interview with FOX News on Tuesday, Barr said Mueller requested that the Justice Department send the letter to limit testimony. In the letter, the Justice Department asked Mueller to not disclose information protected by executive privilege.
Earlier, CNN reported that Mueller's deputy Aaron Zebley is expected to appear next to the special counsel as an adviser. Mueller, according to the report, will be the only one allowed to ask questions. Mueller's testimony on Wednesday is taking place a week after the hearing was originally scheduled for.
Mueller issued the final report on his investigation in April and said the probe found no evidence of collusion between Trump and his campaign with Russian officials in the run-up to the 2016 US presidential election. However, Mueller described ten instances that may constitute obstruction of justice by the US president.
Barr said after reviewing the facts concluded that the evidence developed by Mueller failed to establish that Trump committed an obstruction-of-justice offence.
Trump has repeatedly called Mueller's investigation a political witch hunt. Russia has also denied any claims of interference in the US political system and said the allegations were made up to deflect public attention from actual instances of election fraud and corruption as well as other pressing issues.