She was one of the first contestants to be dumped from this series of the ITV1 primetime.
Her place was resurrected following the controversial exit of Frankie Cocozza. She was seen last night (24 November) to have replaced fellow contestant Misha B to sing the closing key line of the ad's song, ‘Wishing On A Star’.
Amelia Lily was in danger of elimination at the end of last Sunday's show, which drew a peak audience of 13.9 million, but was saved by the public vote after a judges' deadlock forced the decision.
'The X Factor' fans have taken to Twitter to question the decision, with some branding the change a ploy to up Amelia Lily’s chances of dodging a place in the bottom two on this Sunday's show.
Her popularity with voters on the show has dropped since last Sunday's sing-off, with some news sources suggesting that she is due to land in the bottom two this week.
Comments on Twitter included, "Marks and Spencer replaced the end of the advert with Amelia. What about Misha!" and "It’s a fix."
A spokeswoman for Marks and Spencer said: "We have been editing the ad since the beginning [of the elimination stages] and will continue to do so up until the finals."
The ad has already attracted its fair share of controversy, due to its weekly changes to the line-up, which originally included all 16 'X Factor' finalists and still does in some of the group shots.
Marks and Spencer cut Cocozza after he was thrown off the show for boasting about cocaine-fuelled sex sessions. Prior to that, the re-editing of the campaign was not known.
Rainey Kelly Campbell Rolfe Y&R was not available for comment about the re-editing of the ad.
Follow Mark Banham on Twitter @Banham72