Former boxing champ Kell Brook said Thursday he "messed up" ... apologizing to his family and fans after a video showed him appearing to snort white powder at a recent party.
The clip, obtained by The Sun and posted last month, seemed to show Kell at a Sheffield residence leaning over a glass coffee table and putting a line of the substance through a straw and up his nose.
In the footage, Brook -- wearing a Joe Louis t-shirt -- can be heard saying "nice" ... before celebrating with some shadow boxing.