Bornstein, who was Trump's doctor since 1980, told CNN he was in a vehicle with his wife and was speaking with the then-presidential candidate on the phone and Trump was telling him what he wanted the letter to say.
"This is like a celebration for me", said Bornstein, who has previously expressed interest in taking on the role of White House physician.
Bornstein's suggestion that Trump dictated the health assessment comes at odds with his previous claims that he penned the letter in between seeing other patients.
Bornstein explained that Trump called him as he and his wife were driving through Central Park, reading off the language he wanted in the letter.
"He dictated the whole letter".
"His physical strength and stamina are extraordinary", the letter said.
The incident left him "raped, frightened and sad", he said. (Bornstein became Trump's doctor in 1980, when he inherited the real-estate developer as a patient from his father, Dr. Jacob Bornstein.) Bornstein's statement was greeted with a lot of skepticism.
A few weeks ahead of its release, Mr Trump tweeted that Mr Bornstein's medical report would show "perfection".
A person familiar with the episode, speaking to CNN on condition of anonymity, believes Bornstein was presented with a letter from the White House physician's office requesting the release. "It's like the movie "Fargo": it takes the truth and moves it in a different direction".
The doctor has also claimed his offices were "raided" by the President's longtime former bodyguard.
The White House says President Trump's former bodyguard did nothing wrong by taking the president's medical records from his NY doctor's office a year ago.
His previous physician was my father, Dr Jacob Bornstein. "Forget it, you're out", she allegedly said.
Reporter: Bornstein says the president's former bodyguard Keith Schiller, chief of oval operations on the time, a lawyer on the trump group, together with a 3rd man got here into his workplace in February of 2017.
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