Katie, 37, is currently starring as wicked witch Malevolence in Sleeping Beauty and performed two numbers to the bemused crowd last night in Surrey.
The former glamour model was first seen miming along to the Eurythmics hit 'Sweet Dreams' despite her fellow co-stars singing their numbers live.
One audience member tells Reveal: "It was so obvious she was miming to a backing track of her own voice. She'd obviously pre-recorded a version of 'Sweet Dreams' which was then played in the theatre. I think it's fair to say autotune had been used!"
But despite many fans being left disappointed, Katie then opened the second half of the pantomime, at Woking's New Victoria Theatre, with a LIVE performance of Little Mix's No1 hit 'Black Magic'.
The pantomime-goer added: "Yes, it was very obvious she was singing live the second time. Very obvious!"
The Sleeping Beauty programme makes reference to Katie's past singing career, stating: "Katie has been a contestant on the Eurovision Song Contest as well as releasing her own single."
Katie is appearing as the wicked witch until 27 December, before handing over the role to former EastEnders star Anita Dobson.
Anita was in last night's audience to watch Katie and it is clear the script will be changed once the mum-of-five steps down later this month.
During the show, Katie mocked her three marriages and two divorces after Sleeping Beauty had agreed to marry her Prince Charming.
The star - who was previously married to Australian pop star Peter Andre and cage fighter Alex Reid - quipped: "I'm good at weddings...and divorces..."