Pomerantz's letter said that he had quit over the decision by new Manhattan district attorney Alvin Bragg not to move ahead with prosecution of the Republican billionaire.

That decision, he wrote in the letter which the New York Times published in full, was "contrary to the public interest."

"The team that has been investigating Mr. Trump harbors no doubt about whether he committed crimes -- he did," Pomerantz wrote.

The investigation had probed whether Trump fraudulently overvalued multiple assets to secure loans and then undervalued them to minimize taxes.

It was launched by Bragg's predecessor Cyrus Vance, with Bragg taking over the case when he took office in January.

Tacopina also expressed concern over an upcoming book published by Pomerantz, saying it could contain more falsehoods about the former president.

"I strongly admonish you to take these next words seriously: If you publish such a book and continue making defamatory statements against my client, my office will aggressively pursue all legal remedies against you and your book publisher, Simon & Schuster," Tacopina's letter stated.

"Trust me, I will zealously use every possible legal resource to punish you and your publisher for the incredible financial harm that you have caused my clients to suffer," he added.