Online bookmaker Paddy Power is under fire for its latest controversial advert, which is accused of being transphobic.

The ad focuses on the upcoming Cheltenham Ladies Day horseracing event, with a Paddy Power customer saying he "can't wait to see some beauties".

"We are going to make Ladies Day even more exciting by throwing in some beautiful transgendered ladies," the voiceover says, before asking viewers to "spot the stallions from the mares".

A series of women then appear onscreen - some transgender - with the voiceover guessing if they are male or female.

The advert was posted on YouTube, with comments lashing out at its content.

"There's nothing funny about transphobia. It's disgusting to think we live in a society where transphobia is normal," wrote one viewer.

"This is incredibly transphobic. Trans people (trans women in particular) have been made to be the punchlines to jokes over and over again. You'd think by 2012, this wouldn't still be such a problem," another wrote.

Stephanie Stevens, a feminist blogger, has written a letter of complaint to the Irish bookmaker.

"By suggesting that transwomen should be sought out and openly identified as men, your advert not only shows a gross misunderstanding of gender identity, it is inciting viewers to transgender bullying and hatred," she wrote.

Transgender campaigners Trans Media Watch (TMW) also spoke out.

"TMW considers the new Paddy Power advert one of the worst examples of transphobia in this medium for many years," Jenny Kermode, chairwoman of TMW, told International Business Times UK.

"We have recently been looking at options for raising awareness and tackling prejudice within the advertising sector, and we hope to discuss this advert directly with the Advertising Standards Authority as we feel it exemplifies the worst excesses of the industry.

"There are thousands of trans women in Britain. The chances are that you meet them from time to time without ever noticing. Trans women are not trying to imitate other women, they are simply trying to get on with their lives and be themselves."

Kermode also said that the advert puts trans women "at risk" by presenting them as "deceptive" as well as "calling their femininity into question" and encouraging "prejudice against other women who may happen to have masculine features".

"Paddy Power have no right to tell women how they should look, and I trust that their female customers will respond to this advert by finding somewhere else to spend their money," she added.

A spokesman for Paddy Power insisted no offence was intended by the advert.

"We are aware that some people have taken offence to our 'Ladies Day' ad - which wasn't our intention," he said.

"The ad is meant as nothing more than mild-mannered fun in the run-up to the annual Cheltenham Festival.

"In fact, several members of the UK transgender community are cast in the ad, which incidentally was approved by Clear Cast, the UK TV commercial vetting body."

A spokesman for the Advertising Standards Authority said no complaints have been received about the advert so far.