Kym Marsh has criticised Cheryl Cole's controversial decision to sit on the fence and said "a judge's job is to judge".
The Coronation Street star, who rose to fame through a forerunner of The X Factor, said Cheryl "should have voted".
Cheryl's refusal to vote cost contestant Treyc Cohen a place on the show and saved Katie Waissel who was less popular with viewers. If she had voted for Treyc, the decision would have been thrown over to the votes of viewers.
Kym said: "I do think she should have voted. It could potentially have changed the whole outcome, and fans of Treyc are probably fuming.
"You can see why people are annoyed. I do believe she should have chosen because if you're going to be a judge your job is to judge. Whether that's your own acts or someone else's, in my opinion that's what she should have done."
Cheryl said she did not want be forced to choose between Katie and Treyc, who had finished in the bottom two and were both acts for whom she had been mentor.
Kym said it was clear that Katie was proving unpopular with viewers as she had finished in the bottom two on three occasions.
"They should be listening to the people. It's obvious the people will keep putting her in the bottom two. Next week she'll probably be in the bottom two again. It's obvious the people don't want to see her in the final."
Kym was speaking as she backed the Childnet International campaign for safe and responsible internet use. The charity has produced a leaflet to help parents at childnet.com/downloading