If you want it your way at Burger King, hold the horse. That's the advice angry customers have been giving to the fast food giant who has found itself embroiled in a growing scandal.

The fast food retailer was forced to place advertisements in the UK and Irish national press this week in response to growing anger from customers, after it publicly admitted some of its burgers were contaminated in the so-called horse meat scandal.

According to the Guardian, Burger King placed ads in the Sun, Express, Mirror, Star and Times newspapers, apologizing to customers for the 'very small trace levels' of horse DNA found in four samples at Burger King's Irish supplier, Silvercrest.

Burgers taken from restaurants, however, had tested negative. DNA testing is reportedly continuing in Poland, where a supplier used by Silvercrest is thought to be the source of the contamination.

Jaroslaw Naze, Poland's deputy veterinary officer, said a sixth slaughterhouse was now under investigation after it was identified on the labelling of meat under investigation in Ireland. DNA testing carried out in five other slaughterhouses has so far shown no traces of horse protein.

The day after Burger King admitted its involvement in the horse meat scandal, customers at its outlet on London's Euston Road told the Guardian of their concern. Emilie Ashen said: 'It grosses me out. I don't want to be eating horse meat if I'm not told about it – I don't think I'd want to eat horse meat even if I was told about it to be honest.'

Kashyap Raja, 28, said: 'I prefer to have what I've actually ordered in my burger. I usually only buy chicken from Burger King. Chicken is chicken, you know what you are getting. Not so much with burgers.'

Amit Bhadd, 30, added: 'From a vegetarian perspective, I'd be worried if there were meat traces found in something that was supposed to be vegetarian.'

Commenters erupted on social media sites like Facebook and Twitter. @zmama891 Tweeted: 'Burger King.....that is not good!!!!!!! Don't think I will be having a Whopper for a very long time.....here's to homemade burgers.' @Cresean agreed: 'Never have to worry about me eating there EVER again!'

On Burger King's Facebook page, one angry user wrote: 'How does everyone feel about those burgers and the fact that you are also feeding them to your children.'

Another wrote: 'I will NEVER eat in any of your "restaurants" again. FOR SHAME.'

The horse meat scandal will reportedly worry Burger King's management, with signs on Facebook that US-based consumers are questioning their own domestic outlets following the UK scandal.

Burger King's advertisements in the Sun, Express, Mirror, Star and Times newspapers stated: 'You wanted answers. So did we.'

Diego Beamonte, vice-president for global quality at Burger King, told the press the company was 'deeply troubled by the findings of our investigation and apologize to our guests, who trust us to source only the highest quality 100% beef burgers. Our supplier has failed us and in turn we have failed you.'

A spokeswoman for Burger King could not confirm to the Guardian if the company is troubled by the consumer backlash or whether it planned further series of advertisements to keep customers updated on developments.