Editor's note (Aug. 28, 2018, 5:10 p.m.): This article was based on a confirmation from Lanny Davis, Michael Cohen's lawyer. Davis now says he was incorrect when he confirmed this report to NBC News, and has apologized for the error. A corrected version of this article can be found here.

Michael Cohen, President Donald Trump's former personal attorney, asserts that Trump knew in advance about a meeting at Trump Tower in June 2016 between his son Donald Trump Jr. and a Russian lawyer, in contradiction to Trump Jr.'s congressional testimony in May 2017, a knowledgeable source told NBC News on Thursday night.

The source told NBC News that Cohen is willing to inform Robert Mueller, the special counsel investigating alleged Russian interference in the 2016 presidential campaign, that Trump Jr. told his father about the meeting. Cohen's assertion was first reported by CNN.

Lanny Davis, a lawyer for Cohen, said he had no comment.

Trump Jr.'s attorney, Alan Futerfas, indicated that the report was false.

"Donald Trump Jr. has been professional and responsible throughout the Mueller and congressional investigations," he said Thursday night. "We are very confident of the accuracy and reliability of the information that has been provided by Mr. Trump Jr. and on his behalf."

The president's attorney, former New York Mayor Rudy Giuliani, also dismissed the report, saying, "Michael Cohen is not credible."

"Michael Cohen can't be believed unless it's corroborated five times," Giuliani said in a telephone interview. "I talked to the president about this at length before, as well as other witnesses, and it's not true.

"Why would you expect it would be true from someone like Cohen?" he asked. "A lawyer who would tape their own client is a lawyer without any character."

Giuliani's comment was a reference to an audiotape Davis made public on Wednesday of a secretly recorded conversation in 2016 that shows Trump mentioning "cash" in relation to a possible payment involving a former Playboy model who alleges she had a year-long affair with Trump.

Both developments suggest an aggressive move by Cohen, Trump's former "fixer," who is under federal investigation.

In April, the FBI seized eight boxes of documents, four phones, an iPad and several hard drives and storage devices during a raid on Cohen's offices, seeking information about a separate $130,000 payment that he made to porn star Stormy Daniels shortly before the election.

The president has repeatedly claimed that he didn't learn about the meeting until well afterward. Asked about it in an interview with Reuters in July 2017, for example, he said: "No. That I didn't know until a couple of days ago — when I heard about this. No, I didn't know about that."

Asked about the meeting the same month by The New York Times, Trump responded: "No, I didn't know anything about the meeting."