A witness in the George Zimmerman murder trial was forced to admit yesterday that she couldn’t read a letter she supposedly wrote to Trayvon Martin’s mom about his death.

Rachel Jeantel, Martin’s girlfriend, spent more than six hours on the stand, much of it in testy exchanges with defense lawyer Don West.

West asked her to read aloud a March 2012 letter — handwritten in script — that she supposedly wrote and signed with the printed nickname Diamond Eugene.

The letter, in which Jeantel described how she spoke on the phone with Martin, 17, moments before he was shot dead by Zimmerman, was sent to Martin’s mother, Sybrina Fulton.

“Are you able to read that at all?” West asked.

“Some, but not all. I don’t read cursive,” Jeantel said in a whisper, her head bowed.

The disclosure stunned the courtroom in Sanford, Fla.

In a sharper exchange, West suggested that Martin had attacked Zimmerman, a neighborhood-watch member.

“That’s real retarded, sir!” she said. “You don’t know the person.”

In crucial and often combative testimony Wednesday, Jeantel had said that Martin’s last words over the phone were, “Get off! Get off!” and that he had said he was being followed by a “creepy-ass cracker.”

Yesterday, she said she left that detail out of her earlier accounts because she wanted to spare Fulton’s feelings and because she hadn’t been directly questioned about them.

“Nobody asked me,” she said about why she hadn’t mentioned the racially charged “cracker” remark before.

West also asked if the reason Jeantel didn’t call police after Martin’s phone cut off was because she feared he had provoked the fatal confrontation with Zimmerman in a gated community.

“That’s why you didn’t do anything — because Trayvon Martin started the fight and you knew that,” West said.

“No, sir!” Jeantel shot back. “I don’t know what you’re talking about.”With

Jeantel was not asked about reports that dozens of embarrassing postings were deleted from her Twitter account Tuesday night, the eve of her testimony.

The Smoking Gun Web site said the tweets included several references to getting high and drunk, a complaint about “jackass lawyers on my ass” and a photo of what was called her “court nails.”

Fulton left the courtroom late yesterday as the prosecution played a recorded 911 call from neighbor Jenna Lauer and questioned Lauer.

She testified she heard “yelling for help” and someone shout “What the hell are you doing?” But she couldn’t tell who was yelling.