The Advertising Standards Authority (ASA) has received 360 complaints about the ad that posed the question whether racegoers at the Cheltenham Festival were “stallions or mares”.
The ASA is considering complaints that the ad is derogatory towards transgender people.
A spokesman for Paddy Power said it was not the intention to cause offence and that the ad was intended to be “mild-mannered fun”.
Pink News, which claims to be “Europe’s largest gay news service” has led calls for the ad to be investigated. An editorial published online last week said: “Paddy Power may think its advert is just a bit of harmless fun, but it feeds into an environment which dehumanises trans people, adding to the struggles of an already vulnerable minority.”
Paddy Power is no stranger to controversty. It featured Imogen Thomas in ads last May soon after she was at the centre of a media storm around “super injunctions” due to her alleged affair with Manchester United player Ryan Giggs.
An ad for the bookmaker showing a cat being kicked by a blind football player, cleared of being offensive by the ASA, racked up 1,313 complaints in 2010.