Viewers saw Mel, 39, make the tough decision to vote against 31-year-old Cheryl’s act Lauren Platt, 17, in the sing-off.
Despite the fact Lauren stayed, with Stereo Kicks getting the boot, Cheryl couldn’t contain her fury.
The singer made a dig at Mel on screen, saying: “I find it amazing that the critiques have been a certain way over the last few weeks, then it comes to this part and it changes. That amazes me.”
But The Sun reports that’s where their argument started, with Mel furiously mouthing to Cheryl ‘You what?’
KATIE HOPKINS SLAMS STEREO KICKS' MUMS: "ROPEY BUNCH OF FISH WIVES"
The two former girl band members then refused to sit next to, or even look at each other on the Xtra Factor afterwards.
A source said: “What happened between Cheryl and Mel B on Saturday night was real. In Cheryl’s opinion, Mel completely went back on everything she has been saying all series about Lauren and Stereo Kicks.
“Cheryl was livid that Mel turned on her act and the fact she called her out in front of millions shows just how angry she was.
“But Mel is more than capable of standing up for herself and when the cameras stopped rolling she pulled Cheryl up straight away. That led to a really frosty atmosphere between the two of them.”