The former Take That singer spent five weeks preparing for his big performance at the London prizegiving on Wednesday night and cut out sugar and dairy as part of his health kick.
However, speaking on the BRIT Awards red carpet, Williams, 39, insisted the diet was not much fun.
He said: "No candy at the moment. I've given up sugar. 40 is around the corner... I've done 24 pounds in five weeks. No dairy. No fun. No smiles. No laughter. Just horrendous."