In a newly unsealed filing in federal court, Michael Flynn's legal team outlined more than 40 records that may contain exculpatory evidence allegedly withheld by the government -- including a memo from the British government that's said to warn about the credibility of dossier author and FBI source Christopher Steele.

According to the filing, which was unsealed with light redactions, a letter was "delivered by the British Embassy to the incoming National Security team after Donald Trump’s election, and to outgoing National Security Adviser Susan Rice" that "apparently disavows former British Secret Service Agent Christopher Steele, calls his credibility into question and declares him untrustworthy."

Fox News has reached out to the British Embassy and an attorney for Rice for comment about the claim.

Nevertheless, the FBI relied on Steele’s work -- funded by Hillary Clinton's campaign and the Democratic National Committee through opposition research firm Fusion GPS-- to obtain and renew secret surveillance warrants to monitor a former Trump aide. Claims by Steele led investigators to cast a wide net in scrutinizing the Trump team.

Flynn's legal team also sought documentation concerning "former Acting Attorney General Sally Yates, regarding her opinion that the Jan. 24, 2017, surprise interview of Mr. Flynn was problematic and her lack of clarity of the FBI’s purpose in investigating Mr. Flynn."

Flynn was not charged with wrongdoing as a result of the substance of his calls with then-Russian ambassador Sergey Kislyak that were the subject of that interview -- and a Washington Post article published one day before Flynn's White House interview with the agents, citing FBI sources, publicly revealed that the FBI had wiretapped Flynn's calls and already cleared him of any criminal conduct.

Flynn was charged with one count of lying to the FBI for his allegedly equivocal responses to FBI agents during that interview, Flynn told agents "not really" when asked if he had sought to convince Kislyak not to escalate a brewing fight with the U.S. over sanctions imposed by the Obama administration, according to government documents.

DOJ MEMO ALLEGEDLY CLEARED FLYNN OF BEING RUSSIAN AGENT -- AND NOW IT'S MISSING

The FBI, including then-Deputy Director Andrew McCabe, pushed Flynn not to have a lawyer present at what seemed to be a casual meeting at the White House, Flynn later said.

The former national security adviser's lawyers argued in a status conference this week that the case against Flynn should be thrown out, accusing the prosecution of "egregious conduct and suppression” of evidence before and after his guilty plea.

Flynn lawyer Sidney Powell convinced U.S. District Court Judge Emmet Sullivan to schedule a hearing on whether so-called Brady material – evidence helpful to the defense -- was unconstitutionally withheld. The developments come ahead of a scheduled December sentencing date, which Sullivan warned the parties could be moved again.

“We've ... learned that there was a letter dated Jan. 30, 2017, or an internal memo from the Department of Justice, that completely exonerates Mr. Flynn of being an agent of Russia, and that document has not been produced to us yet," Powell said, according to a transcript of Tuesday's hearing obtained by Fox News.

FLYNN LEGAL TEAM ACCUSES PROSECUTORS OF MISCONDUCT

Powell continued: "So they had a lot of information that they didn't disclose. They also have knowledge of a letter from the British Embassy that completely discredits [Trump dossier author] Christopher Steele and undoes the whole Steele dossier debacle."

Fox News learned about the memo's existence last week. Asked by Fox News earlier this week if they were aware of the memo and could provide it, a DOJ official said they would try to identify it.

But responding to Powell's concerns in court on Tuesday, a government lawyer said they were irrelevant, noting they did not accuse Flynn of being a Russian agent in the first place.

"The government has not alleged in any filings in this court or before the court that the defendant is an agent of Russia. That is not part of this case. That is not part of the act that he has pled guilty to. It was lying to the FBI about particular communications involving Russia, not being an agent of Russia," prosecutor Brandon Van Grack said, according to the transcript

Aside from Powell's latest claims in court, Fox News reported on Monday that onetime deputy FBI Director Andrew McCabe told the White House in early February 2017 that the bureau was not considering Flynn for a potential Logan Act prosecution over contacts with Kislyak before Donald Trump was sworn in as president. McCabe was referring to the rarely prosecuted 200-year-old statute that bars American citizens from engaging with a foreign government without authorization from the current U.S. government.

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The records also indicated that Flynn reported, on two separate occasions, in the days leading up to his White House firing, that FBI agents told him the bureau investigation was over or being closed out.

"There is far more at stake here than sentencing," Powell said. "There were stunning failures to produce Brady material, going back to July of 2017."