In a cease and desist letter, obtained by TMZ, Crystal's lawyer calls out Holly for her podcast interview with Hugh Hefner's son, Marston -- ripping her remarks on the episode as "malicious and intentionally misleading."

Holly, Marston and Bridget Marquardt chatted on the May 12 episode of the "Girls Next Level" podcast, and according to the letter ... they insinuated Crystal kept Hugh "doped up" during the final years of his life in order to take advantage of him financially.

The letter adds ... "Nothing could be further from the truth. As we understand it, Ms. Hefner was steadfast in her commitment to Mr. Hefner through his final days, ensuring that he was cared for and received the best medical attention available."

Crystal's lawyer emphasizes the model did not capitalize on Hugh's mental and physical decline ... and says suggesting otherwise "is entirely false and will not be tolerated."

The letter warns if Holly does not stop from making similar remarks in the future, Crystal will be forced to take legal action.

Holly's wasted no time responding to the letter, and in pretty bold fashion ... she announced on social media she wiped her butt with the letter after receiving it. She also sounded off on X, writing ... "Imagine running a free speech foundation and then sending someone a cease and desist for talking."

Crystal's attorney tells TMZ... "Ms. Hefner will not have her name and reputation tarnished by opportunists. We stand by the contents of the letter and will enforce Ms. Hefner’s rights to the fullest."

This is not the first time Hugh's exes have gone head-to-head. Remember, in April, Holly claimed Crystal essentially copied her memoir, 'Down the Rabbit Hole' ... alleging Crystal's writing style was eerily similar to her own.

In her own book, 'Only Say Good Things,' Crystal wrote that filming "Girls Next Door" created a rivalry between herself and Holly, which Holly agreed was likely true.