Monica Crowley, who was selected just weeks ago to serve in a high-profile post on President-elect Donald J. Trump’s National Security Council, has decided against taking the position after allegations that she plagiarized key passages in a 2012 book.

Ms. Crowley, whose name was briefly floated as a candidate for White House press secretary, has been dogged by accusations of plagiarism in recent weeks, beginning with the discovery by CNN that she copied several passages in a book she published with HarperCollins. A later report in Politico unearthed similar issues in her doctoral dissertation.

“After much reflection, I have decided to remain in New York to pursue other opportunities and will not be taking a position in the incoming administration,” Ms. Crowley said in a statement to The Washington Times.

“I greatly appreciate being asked to be part of President-elect Trump’s team, and I will continue to enthusiastically support him and his agenda for American renewal,” she said.