Liam Payne's ex-fiancée Maya Henry tells us she's not surprised to see him spewing "sexist" remarks about his current girlfriend ... she says the former One Direction singer is toxic!!!

Maya, who was engaged to Liam for 10 months before they called things off in 2021, tells TMZ ... Liam's behavior in a recent social media post with girlfriend Kate Cassidy is just the tip of the iceberg -- and she's heard worse things about women come out of his mouth.

The model is referring to the video of Liam and Kate showing off their fits in front of a mirror, with Liam telling Kate ... "You look good. Nice and covered up ... for once."

Liam's ex says she truly feels for Kate, because Maya says she knows "firsthand the kind of toxic environment she's living in currently."

For her part, Kate says she told Liam to make a crack about her wearing more clothes than usual ... and she insists Liam has no problem with her showing a lot of skin in public.

Regardless, lots of folks feel Liam is being super misogynistic here ... and Maya's of that opinion too.

Maya actually wrote a book called "Looking Forward," after her split with Liam -- a novel about a model who falls in love with a pop star and then endures an abortion, abuse, self-harm and eating disorders. Maya says the book is inspired by her relationship with Liam.

Sounds like she wants Kate to read the fictional story, telling us ... "I hope sharing some of this helps others realize that you don’t have to put up with any form of misogyny."