EXCLUSIVE

Blake Griffin's ex, Brynn Cameron, claims the NBA star dumped her ONE WEEK before their wedding date because she wouldn't sign a prenup ... and then jetted to Vegas to party without her.

In her lawsuit, obtained by TMZ Sports, Brynn claims the NBA star made an oral agreement with her back in 2013 that she would quit her job to raise their kids so he could focus on getting a max NBA contract, worth more than $170 mil.

In exchange, Brynn says Griffin "promised explicitly that, whatever the future might hold, he would take care of [Cameron] financially and provide her a home for the rest of her life."

Brynn says she and Blake were slated to tie the knot in St. Barth's on July 28, 2017 -- but one month before the wedding, he demanded she sign a prenup.

Brynn shut him down -- claiming the prenup contradicted the oral agreement they made in 2013.

Blake then called off the wedding 1 week before the date -- and Blake sent a note to guests saying, "Brynn and I take this commitment to each other seriously and want to ensure it is done in the right way."

Instead, Brynn says ... Blake "immediately embarked on a high-profile affair with [Kendall Jenner]. The next weekend, on what would have been his wedding day, he was partying in Vegas."

Byrnn says she and Blake began couples therapy -- but he was still seeing Jenner on the side.

It was then Brynn says Blake stopped paying her "full financial support commensurate with her lifestyle and her share of their joint property."