The White House said in a letter to John Bolton's attorney last week that his upcoming book "appears to contain significant amounts of classified information," some of which is "at the TOP SECRET level."

The memoir can't be published until those details have been deleted, the letter said.

The former national security adviser's book, "The Room Where It Happened: A White House Memoir," is said to undermine Trump's impeachment defense.

Trump and his allies have lashed out at Bolton, characterizing him as part of a "deep state" dead set on ousting the president.

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The White House said in a letter to John Bolton's attorney Charles Cooper last week that the former national security adviser's upcoming tell-all book "appears to contain significant amounts of classified information" and can't be published until those details have been deleted.

The letter, written by Ellen Knight, a National Security Council's records-management employee, said the manuscript for the book, "The Room Where It Happened: A White House Memoir," set to be released in March, includes information classified "at the TOP SECRET level."

Citing an executive order, the letter said that information could "cause exceptionally grave harm to the national security" of the United States.

The letter was dated January 23, three days before The New York Times reported that Bolton's book contradicted the foundation of President Donald Trump's impeachment defense. According to The Times, the book alleges that the president categorically linked Ukraine's military aid to requests for investigations into political rivals.

Bolton and Cooper have maintained that the book doesn't contain any such sensitive information.

Knight wrote that federal law and a nondisclosure agreement Bolton signed to gain access to this information mandated that "the manuscript may not be published or otherwise disclosed without the deletion of this classified information."

Officials are reviewing the manuscript and identifying classified information so that neither Bolton's "ability to tell his story" nor national security is infringed upon, Knight wrote.

On Wednesday, Trump lashed out at Bolton in a flurry of tweets deeming the memoir "nasty & untrue" and alleging that had he heeded Bolton's advice, "we would be in World War Six by now."

Once beloved by the GOP, Bolton has been characterized by Trump and his allies as part of a "deep state" dead set on ousting the president.

Senate Republicans are mulling whether to call Bolton to testify in the impeachment trial.

The White House, Cooper, the literary agency Javelin, which represents Bolton, and Simon & Schuster, the book's publisher, did not immediately respond to Business Insider's requests for comment.

This story has been updated.