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 Trumps misconduct believed to have been included in Boltons soon-to-be-released book, The Room Where It Happened: A White House Memoir.

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

The letter shared by Miller claims the manuscript of Boltons book contains significant amounts of classified information, including information classified as top secret.

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

Bolton is widely seen as a possible key witness in Trumps 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, Trumps political rival.

Several White House officials testified that Bolton disagreed with Trumps 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 Boltons testimony, but on Monday, Utah Sen. Mitt Romney said it seems increasingly likely GOP senators will vote to have Bolton testify.

I think its important for us to hear from John Bolton, Romney said.