The 26-year-old wowed spectators by attending the Royal Wedding in a skinny-fit slick black waistcoat and blazer, with cropped trousers and a top hat.
She tied the look together with a chunky grey tie and black stilettos.
Social media users took to Twitter to share their support for the model’s “bold” move.
One user wrote: “Cara Delevingne in a top hat! She's won! Everyone go home! #RoyalWedding.”
Another commented: “@Caradelevingne just shut it down in this power suit. #RoyalWedding.” Wow Cara Delevingne breaking tradition and wearing a tux! SLAY. ALL. DAY. #TLCRoyalWedding.”
A third added: “Cara Delevingne looking like a boss in a tux and tophat at the Royal Wedding.”
Wow Cara Delevingne breaking tradition and wearing a tux! SLAY. ALL. DAY.#TLCRoyalWedding— Nicole Hensley (@bigMtfudgecake) October 12, 2018
I love loooove Cara Delevingne in tux at Princess Eugenie’s wedding. Absolutely the best dressed.— caorite (@caorite) October 12, 2018
Cara Delevingne was right. Y'all shouldve worn pants#RoyalWedding— BAYANerd (@AstroGwenO) October 12, 2018
Have always been a little indifferent about Cara Delevingne... but that is certainly the way to rock a wedding outfit. 🔥🔥🔥— Sally (@SallyTraffic) October 12, 2018
Cara Delevingne arriving at the #RoyalWedding— M A R I A 💎 (@ImTheSoundtrack) October 12, 2018
Too much charm for today 😍
Cara Delevingne in a mourning suit! Bold choice!— vickster51 (@vickster51) October 12, 2018
Cara Delevingne looking like a boss in a tux and tophat at the Royal Wedding 😍— LEAHDELACRUX (@leahdelacrux) October 12, 2018
Meanwhile a fourth said: “I’m watching the royal wedding cara delevingne looks so powerful what a woman !!”
Another questioned: “Is Cara Delevingne the first woman to attend a Royal Wedding in a suit?”
Delevingne joins a host of other celebrities in attendance including, Demi Moore, Kate Moss and Naomi Campbell.
Another 12000 members of the public, who were selected by ballot will listen to a live broadcast in the ground of the castle.
The Queen's granddaughter will marry Mr Brooksbank, 32, in front of thousands of well-wishers in the same chapel where the Duke and Duchess of Sussex married earlier this year.