John Bolton could face legal challenges as he pushes ahead with a book describing conversations he claims to have had with President Trump while serving as his national security adviser, experts said, setting the conservative icon on a potential collision course with the administration he once served.

Bolton’s book, “The Room Where It Happened: A White House Memoir,” is still scheduled to be released in March, even after the National Security Council warned his attorney last week that it will have to be revised because it contained “significant amounts” of classified material. Bolton’s lawyer has disputed that.

Amid the standoff, details about the contents of his manuscript are continuing to leak out, with the New York Times reporting Friday that Trump directed Bolton in May to call the Ukrainian president and urge him to meet with Trump’s personal attorney Rudolph W. Giuliani.

Trump denied Bolton’s account. “I never instructed John Bolton to set up a meeting for Rudolph W. Giuliani, one of the greatest corruption fighters in America and by far the greatest mayor in the history of NYC, to meet with President Zelensky,” the president said in a statement.

As Trump and his GOP allies have lambasted Bolton, the former national security adviser has sounded a defiant note. During a private appearance in Austin on Thursday, he defended administration officials who testified during the impeachment proceedings.

“The idea that somehow testifying to what you think is true is destructive to the system of government we have — I think, is very nearly the reverse — the exact reverse of the truth,” Bolton said, according to Austin’s KXAN television station.

White House officials declined to comment Friday on whether Bolton has been asked to delete certain portions of his manuscript or whether the administration has been in touch with Bolton’s team in recent days. A spokeswoman for Bolton declined to comment.

A representative of Simon and Schuster, which is scheduled to publish the manuscript, declined to comment.

A person familiar with the discussions, who spoke on the condition of anonymity because of the sensitivity of the dispute, said Bolton’s team expects a lengthy fight over the issue but appears determined to see it through.

Legal experts and former government officials said the White House has several tools available to try to halt or delay publication of Bolton’s book, including the pre-publication review process.

“If the administration simply doesn’t want the manuscript to see the light of day, they could just drag it out far beyond the March publication date,” said Guy Snodgrass, who served as a speechwriter to then-Defense Secretary Jim Mattis.

“My biggest concern is the process is too easily corrupted by political designs,” he said.

A book by Snodgrass, “Holding the Line: Inside Trump’s Pentagon with Secretary Mattis,” was held up for five or six months by the Defense Department last year for a security review — and released only once he filed a suit alleging the department was blocking its publication. One of the people he said who asked him to remove material was Bolton, then the national security adviser, which he agreed to do.

Legal experts said the White House might also challenge Bolton’s account as a violation of executive privilege or national security, subjecting him to possible legal challenges or even criminal prosecution if he proceeds with publication.

“The president has ultimate authority for deciding what is classified and what is not classified, and Mr. Bolton has an uphill battle to convince the president that there’s no classified information in there,” said John Ficklin, former senior director for records and access management at the NSC from 2014 to 2016.

Bolton’s attorney, Charles Cooper, has said his client is confident there is no classified material in his manuscript. Nonetheless, a Jan. 23 White House letter to Cooper warned that the manuscript contained a significant amount of classified material, including some considered top secret.

The letter, written by Ellen J. Knight, the National Security Council’s senior director for records, access and information security management, said Bolton would be breaking his nondisclosure agreement with the U.S. government if he published the book without revisions.

“The manuscript may not be published or otherwise disclosed without the deletion of this classified information,” she wrote.

Cooper had submitted the manuscript to the National Security Council for vetting on Dec. 30.

“Ambassador Bolton has carefully sought to avoid any discussion in the manuscript of . . . classified information, and we accordingly do not believe that prepublication review is required,” Cooper wrote to Knight in a letter accompanying the draft. “We are nonetheless submitting this manuscript out of an abundance of caution.”

In the past, the U.S. government has had mixed success in its attempts to block the publication of books by former officials. Many experts say the system for reviewing manuscripts for potential classified material is dated, opaque and in need of reform.

“The whole prepublication review system is a mess,” said Oona Hathaway, a professor at Yale Law School who previously served in the Defense Department.

“Each agency has its own set of rules. . . . There’s a rationale behind the system, but it’s badly managed, it’s not well organized, it’s not centralized, there are no clear rules, and it is very much open to abuse,” Hathaway said.

Nonetheless, she said, the government has a “Damocles sword” hanging over Bolton, noting that “they could sue him if he doesn’t get the government’s permission, and they could make him give up everything he earns on the book.”

The Supreme Court ruled in 1980 that employees who evade pre-publication review requirements can suffer serious financial repercussions. The case involved former CIA agent Frank Snepp, who described the CIA’s role in Vietnam but failed to submit the publication for review. The court’s decision effectively permitted the government to seize the profits of his book.

Most writings by former government officials — such as op-eds, law review articles and even books — make it through the National Security Council review relatively smoothly. Smaller issues are often resolved via email or in sit-down meetings with the author, according to people familiar with the process.

But people who violate the procedures can face severe consequences. Courts have allowed the government to seize multimillion-dollar advances and royalties from authors who violated their nondisclosure agreements. The government could also bring charges under the Espionage Act, though that’s rare.

In 2016, the Navy SEAL who wrote a best-selling book, “No Easy Day,” about the raid that killed Osama bin Laden, had to pay the federal government at least $6.8 million to avoid prosecution for not getting pre-publication approval for the work.

Karen DeYoung and Alice Crites contributed to this report.