NBC said Tuesday night that it is suspending Brian Williams as its lead anchor for six months as a result of a burgeoning scandal involving exaggerated statements he has made about his reporting.

The network said that the suspension would be immediate and without pay and that Lester Holt would substitute for Williams while he is out.

NBC News President Deborah Turness announced the suspension in a memo to the news staff, saying that Williams had been informed earlier in the day that he would be removed from “NBC Nightly News With Brian Williams,” the nation’s top-rated newscast.

The suspension of Williams, who is also managing editor of the evening newscast, is unprecedented in network news history. Never before has an anchor’s credibility and integrity been called into so much doubt that his employer has resorted to such public disciplinary measures. Dan Rather, the CBS News anchor, left his network under a cloud in 2006 after 44 years, but Rather was not subjected to a lengthy suspension.

“By his actions, Brian has jeopardized the trust millions of Americans place in NBC News,” Steve Burke, NBC Universal chief executive, said in the memo. “His actions are inexcusable and this suspension is severe and appropriate.”

Brian Williams has been suspended from NBC News without pay for six months after several allegations of embellishing stories. But is the problem bigger than the man behind the anchor desk? The Post's Erik Wemple and broadcast news expert Jill Olmsted weigh in. (Gillian Brockell/The Washington Post)

Williams, 55, has not commented publicly since releasing a memo Saturday announcing that he would take “several days” off as questions swirled around statements he has made about his reporting in Iraq, New Orleans and other places over more than a decade.

Turness wrote that Williams “misrepresented” events in a broadcast last Friday about his coverage of the Iraq war in 2003. “It then became clear that on other occasions Brian had done the same while telling that story in other venues,” she wrote. “This was wrong and completely inappropriate for someone in Brian’s position.

“In addition, we have concerns about comments that occurred outside NBC News while Brian was talking about his experiences in the field.”

She added: “As Managing Editor and Anchor of ‘Nightly News,’ Brian has a responsibility to be truthful and to uphold the high standards of the news division at all times.”

The network has a multimillion-dollar investment in Williams, who has been the face of NBC News since taking over for Tom Brokaw in 2004. He was rewarded in December with a new five-year contract that pays him well in excess of $10 million annually. His newscasts last week attracted more than 10 million viewers per night, although some of the audience may have been lured by widespread reports about the controversy surrounding the anchorman.

It’s unclear what future Williams has at NBC, because questions about his credibility are likely to linger. NBC will have to assess whether its image would be tarnished by continuing an association with him as well.

As relatively tough as the sanction against Williams is, some observers think it wasn’t enough. “Anyone else at NBC who engaged in this conduct would have been fired immediately,” said Mark Feldstein, a University of Maryland professor of broadcasting. “Did NBC really complete a thorough investigation of all the allegations of false reporting by Williams in just one week? I don’t think so.”

NBC announced it has launched a probe into anchor Brian Williams's accounts of his reporting during the Iraq War and Hurricane Katrina. Here are a series of stories Williams told of his time spent in New Orleans after the levees broke. (Gillian Brockell and Ashleigh Joplin/The Washington Post)

Williams’s attorney, Robert Barnett, had no comment on his client’s suspension.

Williams’s tumble from the top perch in network news broadcasting came about with startling swiftness.

It was set in motion by his apology to a group of Iraq war veterans who had challenged his reporting about an incident at the start of the war in 2003. In a celebratory report aired on “Nightly News” on Jan. 30, Williams saluted the retirement of a soldier he had befriended after Williams’s helicopter landed in the Iraqi desert. But Williams misstated a key detail: that his helicopter had been damaged by ground fire and was forced to land.

In fact, Williams’s aircraft had not faced an attack, and in his apology, he said he had “conflated” his memory of the event with that of another helicopter.

The admission led to other questions about Williams’s characterizations of his reporting on entertainment programs such as the “Late Show With David Letterman” and “The Daily Show With Jon Stewart.” Reporters and bloggers found other discrepancies in his accounts of events, including during Hurricane Katrina in 2005 and during Israel’s war with Hezbollah in 2006.

Turness said she made the decision along with her boss, Patricia Fili-Krushel, chairman of the NBC Universal News Group, and Burke.

“We felt it would have been wrong to disregard the good work Brian has done and the special relationship he has forged with our viewers over 22 years,” Turness wrote to the staff. “Millions of Americans have turned to him every day, and he has been an important and well-respected part of our organization.

“As I’m sure you understand, this was a very hard decision. Certainly there will be those who disagree. But we believe this suspension is the appropriate and proportionate action.”

Turness quoted Burke, also Comcast executive vice president, as saying: “Brian’s life’s work is delivering the news. I know Brian loves his country, NBC News and his colleagues. He deserves a second chance and we are rooting for him. Brian has shared his deep remorse with me and he is committed to winning back everyone’s trust.”

Holt, also 55, has been a solid and familiar figure on NBC News and its sister cable network, MSNBC, but is generally a lesser-known personality than Williams, who has expanded his audience through dozens of appearances on entertainment programs. Holt has been filling in for Williams since Monday. He also anchors “Weekend Today” and NBC’s weekend newscasts.

The suspension of Williams is a setback for Turness, a British television executive who was brought in by NBC and Comcast in mid-2013 to address the news division’s mounting problems, especially the morning “Today” show’s slipping ratings. Turness hasn’t turned around “Today,” or the Sunday morning “Meet the Press,” which went through its own anchor turmoil last year with the sacking of host David Gregory and his replacement by Chuck Todd.

“Nightly News” had been NBC’s one rock of stability. Without its popular anchor, the newscast could be vulnerable to its nightly rivals, ABC’s “World News Tonight” with David Muir and CBS’s “Evening News,” anchored by Scott Pelley.

The rebranding of Williams’s newscast is already under way. Although a formal decision hasn’t been made, his name will be removed from the title, people at the network said, making it simply, “The NBC Nightly News.”