The Deep Space Deviants, a dance act led by a man dressed as Doctor Who baddie Davros, was buzzed by all four judges.
The bizarre act included Davros "dancing" and singing to songs including Move Like Jagger and Don't You Want Me Baby while wheeling himself around stage.
It is highly unusual for all the judges to stop a semi-final act during a performance.
David Walliams said: "We've had some rubbish on the show over the years, but that beats all of them."
Amanda Holden said: "I thought that was bloody awful. I have to say Davros, honestly I thought your audition was better. I think it's gone downhill."
Simon Cowell pointed out that he had not liked the act in the first place, but had been overruled by the other three.
He said: "Without being rude, that was beyond bad. It was the singing, or actually the not singing ... At least I said no the first time around. You three are responsible."
Davros, played by Paul, complained: "I did not get to show you my legs", before bringing out a pair of fake floppy legs and pretending to make a call through his foot.
Alesha Dixon said: "I honestly think you should come back next year and do stand up comedy, because you were really funny, but the act was awful."
Despite the poor performance, viewers seem to be all for voting to keep him in. Mostly to annoy Simon...