Former acting attorney general Sally Yates is expected to tell the US Congress she expressed alarm to the White House about Donald Trump's national security adviser's contacts with the Russian ambassador.
Sally Yates' evidence on May 8 could contradict the administration's version of events.
Ms Yates is expected to recount in detail her January 26 conversation about Michael Flynn and to say that she saw discrepancies between the administration's public statements on his contacts with ambassador Sergey Kislyak and what really transpired, according to a person familiar with that discussion and Ms Yates's plans to give evidence.
According to the source, she will say that she told White House counsel Don McGahn that she was concerned Mr Flynn's communications with Mr Kislyak could leave Mr Flynn in a compromised position as a result of the contradictions between the public depictions of the calls and what intelligence officials knew to be true.
White House officials have said publicly that Ms Yates merely wanted to give them a "heads-up" about Mr Flynn's Russian contacts, but Ms Yates is likely to say she approached the White House with alarm, according to the source.
"So just to be clear, the acting attorney general informed the White House counsel that they wanted to give a 'heads up' to us on some comments that may have seemed in conflict with what he had sent the vice president out in particular," White House spokesman Sean Spicer said at a February 14 press briefing.
Mr Flynn resigned in February after published reports detailed Ms Yates' conversation with Mr McGahn.
White House officials initially maintained that Mr Flynn had not discussed Russian sanctions with Mr Kislyak during the transition period, but after news reports said the opposite, they then admitted that he had misled them about the nature of that call.
"The issue, pure and simple, came down to a matter of trust," Mr Spicer said.
Mr Flynn was in frequent contact with Mr Kislyak on the day the Obama administration slapped sanctions on Russia for election-related hacking, as well as at other times during the transition, a US official has said.
Ms Yates' scheduled appearance before a Senate judiciary sub-committee, alongside former national intelligence director James Clapper, will provide her first public account of the conversation with the White House.
It will also be her first evidence before Congress since Ms Yates, an Obama administration holdover, was fired in January for refusing to defend President Trump's travel ban.
She was previously due to appear in March before a House of Representatives committee investigating Russian interference in the presidential election, but that hearing was cancelled.