Updated

In the latest round in the war between Donald Trump and a growing list of the heaviest hitters in show business, the real-estate mogul and "Celebrity Apprentice" host fired back at David Letterman over the late-night host's on-air suggestion that Trump is a racist.

In a letter he sent to the gap-toothed comic on Friday, Trump canceled an appearance on "Late Show With David Letterman" and lambasted Dave for saying Trump's prolonged attacks on President Obama had begun to "smack of racism."

"I was disappointed to hear the statements you made about me last night on your show that I was a 'racist,' " Trump wrote to Letterman. "In actuality, nothing could be further from the truth and there is nobody who is less of a racist than Donald Trump."

He continued: "Based upon your statements, and despite the fact that we have always done so well together, especially in your ratings, I am canceling my May 18th appearance on your show.

"Additionally," Trump wrote, "please inform your staff that I will likewise not do the 'top ten' list that they had been trying to set up."

On his show Thursday night, Letterman discussed with guest "Dr. Phil" McGraw Trump's persistent questioning of whether Obama is an American citizen and demeaning his qualifications for getting into Harvard Law School.

"It's all fun, it's all a circus, it's all a rodeo until it starts to smack of racism. And then it's no longer fun," said Letterman.

Letterman also said that if Trump comes on his show again, "he ought to be prepared to apologize." Trump's smackdown of Letterman follows tongue-lashings he gave Jerry Seinfeld and Robert De Niro after they similarly objected to his questioning of Obama's birth certificate.

A rep for Letterman declined to comment.

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