Courtney Burgess and his attorney Ariel Mitchell fired off a nasty response to Christian and Quincy Combs after the brothers sent Burgess and Mitchell a cease-and-desist over the alleged Porter tell-all.

In the legal response, obtained by TMZ, Burgess and Mitchell take issue with Christian and Quincy claiming he is trying to capitalize on Kim's hard-earned fame by selling "Kim Porter Tell It All" ... they say Kim was not an actor or entertainer and her fame was solely tied to being Diddy's "long time paramour."

Cheap shots aside, Burgess says he absolutely has the copyright to the book and says Kim gave him the rights before her death.

As we reported ... the Combs bros say they became the rightful heirs and owners of Kim's intellectual property rights following her November 2018 ... and therefore her purported diary entries belong to them.

However, Mitchell tells TMZ ... Kim and Burgess had a mutual friend who was a music producer back in the day and they met through this person, and Kim told Burgess in a phone call that she was going to give him a copy of her diary/memoir before sending it over to him ... and that's how he got the rights.

We actually spoke to Burgess and Mitchell a few weeks ago in NYC, where they told us they had just gotten done with a federal grand jury and were going to meet with federal prosecutors about the Diddy case.

We're told the feds seized Burgess' phone, which contained a copy of the flash drive data used for the Porter book.

Kim and Diddy's sons also told Burgess they wanted an accounting of any money he's made off the book ... and Burgess says their cease-and-desist is a "desperate ploy" crafted by Diddy to extort money ... and he says this "pathetic attempt illustrates just how dire the financial situation is" for Diddy and his sons.