ITV1 show This Morning breached the broadcasting code when guest Amanda Holden plugged a group of law firms, media regulator Ofcom has ruled.
The star was interviewed about her roles on Britain's Got Talent and in the show Shrek The Musical in July.
When she was asked about what else she was doing, the actress, 40, talked about QualitySolicitors.
She said: "I'm part of this new campaign as well that I wanted to come and talk to you about.
"It's called QualitySolicitors. Basically, if you need a solicitor you can walk into WHSmith.
"It's on the high street now. There are 100 WHSmiths in the country and there is a list that's now recommended by the public, for the public, of solicitors who are kosher, who are not going to rip you off and who can help you and it's completely free. You get advice free."
Under current rules, products and services must not be promoted or given undue prominence in programming, to protect viewers from surreptitious advertising.
If product placement is used, which was not the case on this occasion, a logo has to be broadcast to viewers.
ITV said that Holden's recent involvement with the organisation was of interest to viewers and that, in the context of the interview, "her comments were not overly promotional or presented in such a way that made them unduly prominent".
Ofcom disputed ITV's claim that the presenters appeared to "comment humorously on what she was talking about" saying that the remarks of "one of the presenters appeared to endorse QualitySolicitors".
It ruled that the programme promoted QualitySolicitors, its law firms and WHSmith and was therefore in breach of the code.