The call from Flynn lawyer Robert Kelner to Trump attorney John Dowd came Wednesday evening and is a potentially ominous sign for Trump and his close associates. Before this week, Kelner had been communicating with lawyers for Trump.
The split suggests that Flynn, who has been a top target of special counsel Robert S. Mueller III and his team, may be looking to share information with the prosecutor and his team.
The development was first reported by the New York Times.
Kelner did not respond to a request for comment Thursday.
Jay Sekulow, an attorney for Trump, said, "This is not entirely unexpected."
"No one should draw the conclusion that this means anything about General Flynn cooperating against the president," he said, adding, "It's important to remember that General Flynn received his security clearance under the previous administration."
In complex investigations, lawyers for subjects often enter into agreements that allow them to share information without waiving attorney-client privilege. Such agreements generally include provisions that require the lawyers to immediately end the arrangement if their clients begin discussions with prosecutors or if other developments pose a conflict of interest.
Even if Flynn has begun discussions with Mueller's office, there is no guarantee he will ultimately reach a deal with prosecutors.
Flynn served as director of the Defense Intelligence Agency under President Barack Obama before he was pushed out in 2014 amid criticism of his management style and clashes with other administration officials.
He then established a private consulting firm and gave paid speeches, work that has drawn intense scrutiny from Mueller.
In December 2015, Flynn was paid by the Russian-government-funded television network RT to attend a dinner in Moscow, where he was seated next to Russian President Vladimir Putin.
Even as he became an adviser to Trump's presidential campaign, his consulting firm was paid to promote the interests of the Turkish government. Flynn initially failed to report his payments from either engagement.
Flynn served 24 days as Trump's national security adviser, but was forced to resign after acknowledging that he had secretly discussed sanctions with Russian Ambassador Sergey Kislyak during Trump's presidential transition in December.
Lawyers for the president and senior White House aides had been quietly speculating over the last few weeks that Flynn was under increasing pressure to cooperate because Mueller had signaled his ability to charge his son alongside the father.
Michael G. Flynn, the retired general's son, helped his father with business arrangements and served as chief of staff at his father's consulting firm, Flynn Intel Group.
Barry Coburn, an attorney for Michael G. Flynn, declined to comment.
Tom Hamburger contributed to this report.