Ofcom received nine complaints about Channel 4 programme Racing: The Morning Line, aired on September 15 at 8am during coverage of the St Leger festival 2012.
Concern was expressed over an interview with David Williams, a representative of Ladbrokes, who invited viewers to take part in a special betting offer on the horse Camelot, who was a rare contender for the prestigious triple crown after already scooping the 2,000 Guineas and the Derby.
Williams described the details of the money back promotional offer on a £20 stake, and discussed how it could be accessed.
An on-screen caption during the interview stated: "LADBROKES ST LEGER Money back from the sponsors if Camelot loses."
After being contacted by Ofcom, Channel 4 said that the decision to interview the Ladbrokes representative had been taken by production company, Highflyer Productions, which will be replaced by IMG for the relaunch next year.
It said that the production team considered the offer to be "a significant development within the betting market", and so felt that coverage was justified for editorial reasons.
However, Channel 4 accepted that with hindsight the "delivery of the item was not appropriate", and the decision to broadcast should have been taken with "more thought being given to the intended focus of the interview and consideration given to... issues such as undue prominence".
Ofcom said that the inclusion of the on-screen caption showed that the production team was aware prior to the interview that the Ladbrokes representatives were going to plug the betting offer on air.
It also noted that the The Morning Line presenter Tanya Stevenson did nothing to limit the references, and instead further endorsed the offer by describing it as "phenomenal".
Ofcom said that the overall effect of the interview and on-screen caption was to "promote and endorse the Ladbrokes betting offer", and so it recorded a breach of its broadcasting guidelines.
"Ofcom considered that there was insufficient editorial justification for the inclusion of these commercial references within the programming," said the watchdog.
"Ofcom therefore found these references to be unduly prominent, in breach of Rule 9.5 of the Code."
However, it noted that Channel 4 had not received any money for allowing Ladbrokes to promote the offer, and had taken steps to ensure future references to betting products are compliant with broadcasting rules.
But Ofcom's ruling is an embarrassment for Channel 4 as it comes ahead of the major relaunch of its racing coverage next year, after the broadcaster poached a range of high profile events from the BBC, including the Grand National.
Clare Balding will lead the new-look coverage, but Channel 4 has also been accused of ageism over the decision to drop long-running presenter John McCririck.