The 25-year-old Espresso singer paired the figure-hugging dress with a diamond choker and undone, renaissance-style curls.
The Lose Control singer, 32, is known for his playful red carpet looks; he turned heads when he wore a pearl-encrusted three-piece suit to the Grammys last month.
This is the first year Thirlwall is competing as a solo act at the Brit Awards, with nominations for Song of the Year and Pop Act of the Year.
The Messy singer, who is also performing during Sunday’s show, paired the ensemble with her signature jellyfish cut and chunky jewellery.
The 25-year-old artist paired the gown with the French fashion house’s signature pointed pumps
The gown is not dissimilar to the Jonathan Anderson looks we’ve seen flooding the red carpet recently, including his custom blue feather dress for Sabrina Carpenter at the Grammys.
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Former Pussycat Doll Ashley Roberts, 43, wore a shimmering gold keyhole gown, with wet look hair and and a smoky bronzed eye.
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Bandmate Kimberly Wyatt, 43, took a more vampy approach in an asymmetric black dress with leather detail, paired with sheer tights and burgundy velvet platforms.
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The Brit Awards air on ITV at 8.15pm.