One of the deepest, darkest secrets of Russiagate soon may be unmasked. Even President Trump may be surprised.

Multiple witnesses have told Congress that, a week before Trump's inauguration in January 2017, Britain's top national security official sent a private communique to the incoming administration, addressing his country's participation in the counterintelligence probe into the now-debunked Trump-Russia election collusion.

Most significantly, then-British national security adviser Sir Mark Lyall Grant claimed in the memo, hand-delivered to incoming U.S. national security adviser Mike Flynn's team, that the British government lacked confidence in the credibility of former MI6 spy Christopher Steele's Russia collusion evidence, according to congressional investigators who interviewed witnesses familiar with the memo.

Steele, of course, was the political opposition researcher-turned-FBI informant whose dossier the FBI and Obama Justice Department used to justify spying on the Trump campaign in the final days of the 2016 election cycle. The dossier was funded by Fusion GPS, a research firm hired by Hillary Clinton's campaign.

Congressional investigators have interviewed two U.S. officials who handled the memo, confirmed with the British government that a communique was sent and alerted the Department of Justice (DOJ) to the information. One witness confirmed to Congress that he was interviewed by special counsel Robert Mueller about the memo.

Now the race is on to locate the document in U.S. intelligence archives to see if the witnesses' recollections are correct. And Trump is headed to Britain this weekend, where he might just get a chance to ask his own questions.

"A whistleblower recently revealed the existence of a communique from our allies in Great Britain during the early days of the Russia collusion investigation," Rep. Mark Meadows (R-N.C.), a member of the House Oversight and Reform Committee, told me.

"Based on my conversations with that individual, and the credible timelines that are supported by other events, I made a referral to Attorney General William Barr and Inspector General Michael Horowitz for further investigation," he added. "There now is overwhelming evidence to suggest that on multiple occasions the FBI was warned that Christopher Steele and the dossier had severe credibility issues."

The revelation of a possible warning from the British government about Steele surfaces less than a month after a long-concealed document was made public, showing that a State Department official in October 2016 met with Steele and took notes that raised concerns about the accuracy of some information he provided.

Those notes, as I have written, quoted the British operative as saying he had a political deadline of Election Day to make his information public and that he was leaking to the news media - two claims that would weigh against his credibility as an FBI informant. They also flagged a piece of demonstrably false intelligence he provided.

The British Embassy in Washington did not return a call or email seeking comment. Grant, who left his post in April 2017, did not respond to a request for comment at the university where he works. His former top deputy, Paddy McGuinness, declined comment.

Flynn's lawyer, Robert Kelner, also did not return a call, email and text seeking comment.

A source familiar with Flynn's account, however, told me the former Trump national security adviser has no recollection of receiving the British communique or what might have happened to it, meaning that President Trump likely was not told about it.

Flynn first heard about the memo when Mueller's team questioned him about it last year during debriefings after reaching a plea bargain on a charge of making false statements to the FBI and a cooperation agreement with the special prosecutor, the source said.

Mueller's team apparently learned about the memo from some of Flynn's former national security team members, the source said.

Congressional investigators say one former Flynn team member approached them recently as a whistleblower and disclosed the existence of the communique because the person believed it was relevant to the ongoing review of the FBI and intelligence community's conduct in the Russia probe.

The whistleblower told Congress he personally delivered the memo to Flynn on Jan. 12, 2017, was aware of its content about Steele and later ensured the document was sent for preservation in the national security archives of the Trump transition team, the investigators say. The whistleblower also claimed to overhear Flynn's team discussing the memo.

The investigators interviewed a second former National Security Council staffer who claimed to have read the memo in Flynn's office. That person, who requested anonymity because he isn't authorized to talk to the press, told me in an interview that the document contained an explanation from Grant that British authorities assisted the early U.S. investigation into Trump-Russia collusion and later concluded Steele's intelligence was unreliable.

"The message was clear: the Brits were saying they may have done some stuff to assist the investigation that they now regretted after learning the whole thing was based on information from Steele," the former U.S. official told me. "They wanted Trump's team to know they did not think Steele's information was credible or reliable.

"They also wanted Trump to know whatever they had done, they did only at the Americans' request and didn't want it to get in the way of cooperating with the U.S."

Congressional investigators say they have created a timetable of who saw and handled the document in Flynn's office and confirmed with a British government official that Grant sent a memo to Flynn in January 2017, though the British would not discuss its content with congressional investigators.

The information has been turned over to Barr and Horowitz, who are investigating whether FBI, DOJ and intelligence agency officials misused their spy powers or misled a federal court when securing a Foreign Intelligence Surveillance Act (FISA) warrant to monitor former Trump campaign adviser Carter Page.

The congressional investigators believe the communique may have been prompted by BuzzFeed's unexpected publication on Jan. 10, 2017, of Steele's unverified dossier.

American media were engulfed in the budding scandal over whether Trump and Russia colluded to hijack the 2016 election - something Mueller has concluded was not proven by the evidence - and the Brits likely wanted to distance themselves from the document and Steele, investigators believe.

Shortly before Grant's communique arrived, a subordinate national security official in Britain sent a shorter message to Flynn's office. That email didn't address Steele's credibility but communicated that the British had nothing to do with leaking or reacting to the dossier.

A source familiar with Flynn's account said the Grant memo would have arrived just as the national security adviser and president-elect were consumed with standing up a new government, a week before Inauguration Day.

Soon Flynn would be engulfed in new revelations, when intelligence community intercepts of his conversations with the Russian ambassador were leaked to the media.

If the British memo exists, it was never shared with the House Intelligence, House Judiciary, House Oversight and Reform or Senate Judiciary committees, despite their exhaustive investigations into the Steele dossier, congressional investigators told me. These investigators learned about the document in the past few weeks, setting off a mad scramble to locate it and talk to witnesses.

If the witnesses' recollections are correct, the British communique could become one of the most significant pieces of evidence to emerge in the investigation of the Russia-collusion investigators.

It would mean that Trump was never told of the warning Flynn's team received, and that the FBI and DOJ continued to rely on Steele's uncorroborated allegations for many months as they renewed the FISA warrant at least two more times and named Mueller as special prosecutor to investigate Russia collusion.

Former House Intelligence Committee Chairman Devin Nunes (R-Calif.), whose staff has been fighting unsuccessfully to gain access to the British communique, told me Wednesday its public release would further accentuate "that the FBI and DOJ were dead wrong to rely on the dossier in the Russia investigation and to use it as a basis to spy on Americans."

John Solomon is an award-winning investigative journalist whose work over the years has exposed U.S. and FBI intelligence failures before the Sept. 11 attacks, federal scientists' misuse of foster children and veterans in drug experiments, and numerous cases of political corruption. He serves as an investigative columnist and executive vice president for video at The Hill. Follow him on Twitter @jsolomonReports.