Donald Trump's former close adviser Steve Bannon has told the congressional panel investigating last year's January 6 attack on the U.S. Capitol that he is ready to testify, a change of heart days before he is due to be tried for contempt of Congress.

In a letter to the committee, Bannon's lawyer Robert Costello, wrote to say the former president would waive the claim of executive privilege which Bannon had cited in refusing to appear before the committee.

Bannon, a prominent figure in right-wing media circles who served as Trump's chief strategist in 2017, is scheduled to go on trial July 18 on two criminal contempt charges for refusing to testify or provide documents.

The letter from the lawyer said Bannon preferred to testify publicly, but Representative Zoe Lofgren, a committee Democrat, told CNN that ordinarily the committee takes a deposition behind closed doors.

"This goes on for hour after hour after hour. We want to get all our questions answered. And you can't do that in a live format," Lofgren said. "There are many questions that we have for him."

Throughout the House of Representatives committee hearings, videotaped snippets of closed-door testimony by witnesses under oath have been shown to the public.

Trump has been chafing that none of his supporters have testified in his defense at the committee hearings which, separate from the trial, are focused on the attack by Trump supporters seeking to stop the certification in Congress of Trump's defeat by Joe Biden in the November 2020 election.

In a letter from Trump to Bannon, Trump said he was waiving executive privilege because he "watched how unfairly you and others have been treated."

The House panel is due to hold public hearings on Tuesday and Thursday this week.