The White House has issued a formal threat to former national security adviser John Bolton warning him against publishing an upcoming book that alleges Trump abused the power of the presidency, according to a CNN report.

The warning ― which CNN says came in the form of a letter ― follows a New York Times report outlining key allegations of Trump’s misconduct believed to have been included in Bolton’s soon-to-be-released book, “The Room Where It Happened: A White House Memoir.”

See the letter, via Associated Press reporter Zeke Miller, below:

WH LETTER to Bolton lawyer claims his book as information classified at Top Secret level. Bolton and lawyer had maintained they didn't believe the draft contained any classified info. pic.twitter.com/TM395h7GHe — Zeke Miller (@ZekeJMiller) January 29, 2020

The letter shared by Miller claims the manuscript of Bolton’s book contains “significant amounts of classified information,” including information classified as “top secret.”

Trump posted multiple derisive tweets about Bolton on Wednesday morning, criticizing Bolton’s time working for the administration and, without evidence, claiming the former national security adviser’s book is “untrue.”

For a guy who couldn’t get approved for the Ambassador to the U.N. years ago, couldn’t get approved for anything since, “begged” me for a non Senate approved job, which I gave him despite many saying “Don’t do it, sir,” takes the job, mistakenly says “Libyan Model” on T.V., and.. — Donald J. Trump (@realDonaldTrump) January 29, 2020

....many more mistakes of judgement, gets fired because frankly, if I listened to him, we would be in World War Six by now, and goes out and IMMEDIATELY writes a nasty & untrue book. All Classified National Security. Who would do this? — Donald J. Trump (@realDonaldTrump) January 29, 2020

Bolton is widely seen as a possible key witness in Trump’s ongoing impeachment trial, which centers on allegations Trump withheld aid from Ukraine to pressure Ukrainian officials to investigate pro-Russia conspiracy theories and former Vice President Joe Biden, Trump’s political rival.

Several White House officials testified that Bolton disagreed with Trump’s conduct at the time and actively worked to stop it. On Jan. 6, Bolton announced he was willing to testify if called during the impeachment trial currently happening in the Senate. Trump and some of his allies in Congress have argued against Bolton’s testimony, but on Monday, Utah Sen. Mitt Romney said it seems “increasingly likely” GOP senators will vote to have Bolton testify.

“I think it’s important for us to hear from John Bolton,” Romney said.