The X Factor is to be investigated by TV watchdog Ofcom over concerns that it encouraged viewers to download songs by guest stars Diana Vickers and Michael Buble.
The regulator is to examine whether the show broke rules in the broadcasting code on references made to commercial products.
An Ofcom spokesman said: "We are investigating The X Factor for potential breaches of our rules prohibiting the promotion of products in programmes."
The media regulator is to examine two episodes of the ITV1 show.
On October 17, when Vickers - a former X Factor contestant - performed, host Dermot O'Leary told viewers: "If you want to download Diana's single, the details are at ITV.com/XFactor. He made the same remark on the night of Canadian star Buble's performance on October 24.
Under broadcasting rules, products and services must not be promoted in TV programmes.
The rule does not apply to programme-related material, which means that the practice is legitimate when it concerns X Factor contestants but not when it relates to the promotion of guest acts.
The investigation, launched after Ofcom officials noted the references, is the latest controversy to hit the show. On Tuesday, Simon Cowell defended the X Factor from claims that Sunday's results show was a fix, saying: "We haven't done anything wrong."
A spokeswoman for the show said: "We are aware of the Ofcom investigation. It relates to an on-air reference on The X Factor regarding downloads of a guest artist on the show which were not available on the website.
"This reference was as a result of a script error."