Gerald Posner holds a copy of his book Miami Babylon: Crime, Wealth and Power - a Dispatch from the Beach. He admits taking portions from another writer's book without attributing the work. (Alan Diaz/Associated Press)

Gerald Posner, who resigned from the news website The Daily Beast over allegations of plagiarism, has admitted portions of his most recent book are similar to those from another writer's book.

Posner stepped down in February as the top investigative reporter for The Daily Beast when a Slate.com writer compared Posner's pieces with material from the Miami Herald.

Posner said he had "inadvertently" copied passages from some files and forgot to attribute them.

Now, the award-winning author of 10 other books, including Case Closed and Secrets of the Kingdom, says he used some texts from Frank Owen's Clubland without attribution for his book Miami Babylon: Crime, Wealth and Power — A Dispatch from the Beach.

"If you use something from another book, a statement from another book, it needs to be in quotations, or if you take something and put it in your own syntax and grammar, you still need to cite it," Posner said Wednesday in an interview with The Associated Press.

"I do think that the Frank Owen situation may be unique for me. Without going through every line I can't be 100 per cent sure, but I think that is the only case."

Posner says he will go through his book to check the information.

Miami Babylon is a nonfiction piece that examines the gangster and drug underworld of Miami Beach, Fla., and how it is intertwined with the city's business and political circles.

Meanwhile, Simon & Schuster publisher David Rosenthal released a statement in response to Posner's declaration: "We are reviewing the situation and discussing the issues with the author."

Posner says he scanned loads of documents and books he used as research for Babylon and didn't bother to mark where they came from. And since the book took several years, he lost track of the origins of the material.

The writer says he also spent countless hours doing original interviews and digging through court cases and minutes of meetings.

With files from The Associated Press