The drama, set in the 1840s and directed by God's Own Country's Francis Lee, is expected to be released in late 2020.
Ammonite follows Mary Anning, an impoverished paleontologist based on the Southern English coastline of Lyme Regis, and her relationship with rich tourist Charlotte.
Lines uttered with sweeping intensity include: "I don't want to be alone"; "you were the most fascinating person there tonight, and I think the most beautiful" and "I don't want to go back to the life I had before you".
We think they're into each other.
The hotly-anticpated movie will close the BFI Film Festival on Saturday 17 October 2020.
"That they are of the same sex is never addressed or explained"
Speaking to Vanity Fair in September about the real-life Anning, Winslet said: "It’s not supposed to be a biopic and there really isn’t any evidence at all to suggest that she had relationships with women or with men.
"This was Francis Lee’s interpretation. With Ammonite, Francis created a story about two people who fall in love. That they are of the same sex is never addressed or explained - it just is, pure and simple.
The film explores a same-sex relationship in 1840s England (Picture: YouTube)
"This storytelling is so crucial to the progression and the evolution of the way audiences in the world view LGBTQ people and their relationships - by telling these stories that normalize and express same-sex love without hesitation or fear or secrecy. To be able to play this character who shows that level of affection and expression for someone of the same sex has been one of the most joyful experiences of my career."