The former model has become a fan of the ITV1 competition since Twitter followers brought show favourite Susan Boyle to her attention last weekend.
Steel was given a second chance to perform 'Edelweiss' in last night's semi-final after becoming emotional and nervous during her first attempt.
Writing on Twitter, Mills commented: "Caught BGT and loved Aidan the 12-year-old dancer, heartbreaking watching Hollie."
Responding to claims that the youngster's tears were fake, she insisted: "I don't think it's a fix - a 10-year-old can't fake it like that. She was genuinely overwhelmed. Anyway, Aidan was incredible."
The Britain's Got Talent final airs from 6.45pm tonight on ITV1.