Spotlight UK allowed a casting call from Milka, the Swedish chocolate company, to go on its website appealing for a prepubescent girl who is not overweight and does not have red hair.
It specified it wanted "no children over 4ft 4ins", "no overweight children", "she must NOT have reached puberty" and she must be "beautiful and angelic".
The casting call for the chocolate bar's Christmas advert also said: "She can be aged 9-12. If she is 12 she must be very small and still be childlike."
The advert sparked outrage on social media with performer Helen Raw and celebrities including comedian Kathy Burke and Baroness Tanni Grey-Thompson criticising it.
Kathy Burke said: "Imagine being the kids that don't get the job - you're just not beautiful enough I'm afraid, my darling, now off you pop and be riddled with insecurities for the rest of your life."
In a statement posted online, Spotlight UK said the casting call had been "totally unacceptable".
It said: "On Thursday one of the casting directors who use Spotlight's platform published a casting call - or a 'breakdown' as they're known in the trade - on our platform which absolutely did not meet our high standards but nevertheless slipped through the net.
"When our attention was drawn to this we were slow to take action and even then the steps we initially took fell far short of what is expected of us by Spotlight members, casting directors and our own team.
"Breakdowns like this are, quite simply, totally unacceptable and we should never have allowed it to be published.
"However, it was published, and when we were told about it we should have taken it down immediately rather than try to amend it to be more acceptable.
"We did eventually sort things out but we know this should never have happened in the first place and we apologise unreservedly that it did."
The statement added: "We made a serious error and we should have done much, much better. We're sorry."
Mondelez International, which makes Milka, said the notice had not been approved and that a review of the incident was under way.
In a statement, a spokesman said: "We'd like to thank people for bringing this casting notice to our attention.
"We take our advertising responsibility very seriously and this is not representative of the brief we shared with the casting agency and does not meet our high standards.
"We would never approve the use of such a notice, and are urgently reviewing the situation with Spotlight UK to understand how and why it has happened."