5:39 PM PT -- Halsey replied to Britney, sending the love right back ... saying on X, "you were the first person who ever made me realize what it means to feel inspired. And you continue to inspire me everyday."

Halsey's new song, "Lucky" is not a hit with Britney Spears -- or so it appeared based on an angry post accusing Halsey of stabbing her in the back ... a post she almost immediately deleted.

The new music video, directed Gia Coppola, showcases the highs and lows of Britney's career ... with a nearly nude Halsey portraying the pop princess. The song also features a sample of Brit's song by the same title.

Hours after the video dropped ... Britney took to X -- where she has more than 54 million followers -- to blast Halsey for making her feel "harassed, violated and bullied."

In fact, she was so pissed about the depiction of her life, she was contacting her lawyers because she said it felt "illegal and downright cruel."

It's a detailed, and seemingly well thought out response -- but less than an hour later, Britney disavowed the post, saying, "That was not me on my phone!!! I love Halsey and that's why I deleted it 🌹!!!"

The initial harsh reaction had to come as a surprise to Halsey, who previously said she got Britney's blessing to repurpose the pop icon's 2000 hit.

For now, Britney's saying she's cool with Halsey, the song and the music video.

We've reached out to Halsey for comment ... so far, no word back.