Buzzfeed, the cheeky Web site that has soared in popularity with a mix of throwaway lists and hard-news reporting, has owned up to an ethical breach as old as journalism itself.

In an apology published late Friday night, editor Ben Smith acknowledged that one of the site’s most prolific writers, Benny Johnson, had plagiarized the work of others 40 times in some 500 articles and posts. Johnson has been fired, Smith said.

Johnson’s plagiarism began coming to light on Wednesday after Twitter users and a blog called “Our Bad Media” began reporting instances in which he took other’s work without crediting his sources.

Smith initially stood by Johnson, calling him a “deeply original writer” in response to several scattered instances in which he failed to attribute material to its original source.

But the media blog later found that Johnson had used the same wording that had appeared in several Wikipedia entries, and that he had copied verbatim from, among others, a Heritage Foundation report and articles in the National Review, U.S. News & World Report and the New York Times in Buzzfeed pieces last year.

After an internal review of Johnson’s work, Smith found that Johnson’s transgressions went much deeper. “We have found 40 instances of sentences or phrases copied word for word from other sites,” Smith wrote in his apology, without citing any specific posts or their original sources. He called Johnson a friend and colleague “and, at his best, a creative force.”

Smith added, “Plagiarism, much less copying unchecked facts from Wikipedia or other sources, is an act of disrespect to the reader. We are deeply embarrassed and sorry to have misled you.”

Posts with plagiarized material will be corrected, with an editor’s note attached to each, Smith wrote.

Buzzfeed, which was founded seven years ago, has grown into one of the most heavily trafficked news and information sites on the Internet. It popularized “listicles” — articles in the form of a list — and the use of short video snippets to illustrate a theme, typically on frivolous subjects. It has since branched out to more conventional, and more substantial, journalism on politics and foreign affairs, produced by dozens of seasoned reporters in bureaus around the world.

Smith declined to comment when contacted after posting his apology, but said he may have more to say on Saturday. Johnson could not be reached.

The media blog also noted several tweets by Johnson earlier this week, calling out another publication, the Independent Journal Review, for apparently using language from one of his articles. “Repeat after me,” Johnson tweeted, “copying and pasting someone’s work is called ‘plagiarism.”

Buzzfeed began adding notes to Johnson’s work on Saturday, including one on a post about the similarities between some of President Obama’s actions and those of President George W. Bush. It reads, “This post has been corrected to remove phrasing that was copied from The Hill. BuzzFeed takes its responsibility to readers very seriously, and plagiarism is a major breach of that responsibility. Please read our apology to readers.” A link to the editor’s note by Smith follows.

Johnson, who is based in Washington, responded Saturday morning to his firing by tweeting, “To the writers who were not properly attributed and anyone who ever read my byline, I am sincerely sorry.”