The band were seen singing their new single Little Things on the stage in the studio at the end of the show, with host Dermot O'Leary giving them the thumbs-up at the end of their performance.
But it later emerged that the segment was pre-recorded last month - and that Dermot was giving the thumbs-up to an empty stage.
The Mirror newspaper reports that the show's judges even wore the same clothes for the pre-recorded performance, so as not to shatter the illusion for viewers of the show - which had been billed at the start as "live from London".
Although Dermot didn't actually say the band was live, he gave no indication to viewers that the performance was on video either.
He said: "We have been celebrating the best of British tonight and they don't come any bigger or any more British than our next special guest.
"They have conquered the world and it all began for them here. We're the proudest parents on the planet as we welcome back with their new single – One Direction."
Members of the band, who had performed live on The X Factor USA earlier in the week, were papped in LA at the time that they were on TV in the UK - letting the cat out of the bag.
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