The White House has issued a formal threat to former U.S. National Security Advisor John Bolton over his book, "The Room Where It Happened: A White House Memoir". 

According to the White House, the unpublished manuscript of Bolton's book contains "significant amounts of classified information" that could be harmful to the country's national security and therefore, cannot be published in its current form.

The letter from the White House National Security Council to Bolton's attorney, Charles Cooper, and seen by Reuters, said the manuscript contained some material that was considered "top secret" and could reasonably be expected to cause "exceptionally grave harm" to U.S. national security if disclosed without authorization.

"Under federal law and the nondisclosure agreements your client signed as a condition for gaining access to classified information, the manuscript may not be published or otherwise disclosed without the deletion of this classified information," the letter said.

The letter, which was sent via email to Cooper, was dated January 23.

The manuscript has upended the impeachment trial against Republican President Donald Trump, for whom Bolton worked.

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The New York Times reported on Sunday that Trump told Bolton in August he wanted to continue to freeze 391 million U.S. dollars in security aid to Ukraine until officials there helped with investigations into Democrats, including former Vice President Joe Biden, a Democratic presidential candidate, and his son, Hunter.

That revelation has bolstered Democrats' push to call Bolton as a witness in the impeachment trial, in which Trump is accused of abusing his power over his dealings with Ukraine and of obstructing Congress.

The letter, signed by Ellen Knight, the senior director for records, access and information security management, said Bolton's manuscript was still being studied.

"The manuscript remains under review in order for us to do our best to assist your client by identifying the classified information within the manuscript," it said. "We will do our best to work with you to ensure your client's ability to tell his story in a manner that protects U.S. national security."

(With input from Reuters)