12:10 PM PT -- A rep for Firerose tells TMZ … "Billy Ray’s story keeps evolving and this filing is simply another abusive tactic to distract away from the serious, true allegations brought against him, backed by an abundance of evidence. The only individuals supporting his false claims are those on his payroll, whose credibility is questionable."

Then went on to say ... "The audacity to question a BRCA-positive women’s [sic] decision to undergo a preventative surgery further shows the kind of cruel manipulation Firerose was subjected to. What Billy Ray fails to recognize is when you have a decades-long history of emotional and psychological abuse, people start to take notice. The truth will come to light."

Billy Ray Cyrus is pushing back against his estranged wife's claim he abandoned their marriage just one day before her scheduled double mastectomy ... telling a different story.

The country star filed a legal response to Firerose's allegations Monday ... claiming her surgery was actually scheduled for June 6 last year, not May 24 as she alleged. What's more, he alleges she set it for this date as a calculated move for her to tarnish his career by threatening to attribute their split to her double mastectomy.

He claims she said to him, "If you even think about divorcing me right now, I will tell everyone that you did it because of the double mastectomy and your career will be over."

BRC also asserts in the docs that Fireroses' procedure was elective, contradicting her claim the surgery was highly recommended by doctors.

He goes as far as to allege she didn't have cancer during their time together, nor did she show any early signs of cancer. He also claims he only learned about it after their marriage.

He acknowledges Firerose was a carrier of the BRCA1 gene linked to breast cancer -- but he believes she knew about the gene 4 years before they began dating ... that's what he's alleging in his response anyway.

In a further rebuttal to Firerose's claims, Billy denies allegations he coerced her into indefinitely postponing the surgery or that he promised to cover any expenses not covered by insurance.

Of course, this contradicts Firerose's own claim -- namely, that she was under the impression Billy would finance the procedure, only to have the rug pulled out from under her. According to her, this left her scrambling to pull finances together.

We broke the story ... BRC rushed to court in May seeking to scrap his marriage entirely -- and while he cited marital misconduct then ... he didn't clarify the specifics, at first.

But given the legal back-and-forths since then ... it looks like this divorce is getting ugly fast.