Stephen Miller, Donald Trump’s former top adviser, will give testimony to the January 6 committee today, according to a source familiar with the matter.

The reported cooperation of Miller is further evidence that the House investigation into the Capitol riot is lapping at the doors of the Trump Oval Office, after the former president’s daughter and son-in-law, Ivanka Trump and Jared Kushner, both former senior White House advisers, gave their own testimony in recent weeks.

According to two other sources cited by the Associated Press, it is unclear if Miller will appear in person or virtually before the nine-member bipartisan panel.

The fact he is appearing at all is a significant development, however, and probably another major blow to Trump’s efforts to shield information about his movements on the day of the insurrection and subsequent efforts to overturn the presidential election he lost to Joe Biden.

Miller, considered Trump’s top aide through the entirety of his single term in office, has fiercely resisted previous efforts to get him to testify after receiving a subpoena in November. At the time, Bennie Thompson, the Mississippi Democrat who chairs the panel, said Miller had “participated in efforts to spread false information about alleged voter fraud”, the basis of Trump’s big lie that his election defeat was fraudulent.

How cooperative Miller will be in terms of the testimony he has to offer remains to be seen. It is likely that Miller’s decision to appear was prompted by last week’s House vote to hold former Trump advisers Peter Navarro and Dan Scavino in contempt for their refusal to comply with their own subpoenas.

Miller’s testimony – if he is cooperative – could be some of the most valuable and compelling evidence the January 6 inquiry will have collected to date about Trump’s involvement in the deadly insurrection.

Miller was ever-present at Trump’s side throughout his administration, a supremely loyal and focused character credited as the mastermind of some of the most controversial and harshest policies it enacted.

An extremist known for his white nationalist and far-right views, Miller was central to almost every decision the former president made while in office, as well as ultra-hardline immigration policies Trump would probably have enacted had he won a second term.

It is that loyalty to his old boss, and to Trumpism itself, that has analysts wondering if Miller will in fact be forthcoming or will instead plead the fifth amendment to questioning. There is already speculation that Miller’s agreement to appear – which neither he nor the panel has yet confirmed – was simply an exercise in avoiding the fate of Scavino and Navarro.

Miller’s appearance tightens the committee’s focus in the final stages of its investigation on the inner circle of Trump, who has vociferously pushed the big lie that his 2020 election defeat was fraudulent. The former president’s actions on the day of the insurrection and afterward have been under scrutiny, most recently a revelation that calls he made on 6 January were hidden from the official log.

The inquiry has also looked into an illegal scheme allegedly pushed by Trump and his supporters to put forward fake electors to overturn Joe Biden’s victory in the electoral college.

The panel has said it will probably hold public hearings this spring, and a report is expected before this year’s midterm elections. Polling shows Republicans in a strong position to seize a majority in the House, at which point most observers believe they would shut the inquiry down if it is still ongoing.