McDonald’s is once again having to explain itself, yet this time it has nothing to do with its menu items but the actual physical menu itself.

An investigation into the burger chain’s touchscreen menus in the UK has found that every single one showed traces of human faeces after being swabbed.

The testing was done by London Metropolitan University’s school of human sciences and news site Metro.co.uk and each one tested positive to coliform bacteria – faeces or sewage contamination.

Some of the machines even showed signs of listeria contamination that can cause miscarriages and stillbirths in pregnant women.

It is believed that the bacteria is spread by customers not washing their hands after visiting the toilet or covering their nose when they sneeze.

For its part, McDonald’s has said it says the screens are cleaned daily with disinfectant.

London Metropolitan University’s Senior lecturer in microbiology, Dr Paul Matawele, told Metro.co.uk: “We were all surprised how much gut and faecal bacteria there was on the touchscreen machines.

“Touchscreen technology is being used more and more in our daily lives but these results show people should not eat food straight after touching them, they are unhygienic and can spread disease.

“These bacteria can be on touchscreen machines for days on end.

“Seeing Staphylococcus on these machines is worrying because it is so contagious. It starts around people’s noses, if they touch their nose with their fingers and then transfer it to the touchscreen someone else will get it, and if they have an open cut which it gets into, then it can be dangerous,” he said

A McDonald’s spokesman said of the findings: “Our self-order screens are cleaned frequently throughout the day with a sanitiser solution. All of our restaurants also provide facilities for customers to wash their hands before eating.”

The global burger chain has reportedly spent millions of dollars rolling out its self-service kiosks in global markets after first trialling them in Australia with the now abandoned Create Your Taste menu four years ago.

In the US, franchisees were required to fork out up to $US125,000 per restaurant installing the machines, according to Business Insider.

McDonald’s American CEO Steve Easterbrook earlier this year said the self-service kiosks were delivering results, namely by increasing average order size per customer.

“What we’re finding is when people dwell more, they select more,” Easterbrook told CNBC.

“There’s a little bit of an average cheque boost. If you think about only two years ago, if you were a customer there were two ways you can get served at McDonald’s. You walked to the front counter and line up and take your drink and find a table or you go through the drive through.

“We’re introducing many options. They can order through mobile, they can come kerbside and we’ll run it out as well as the existing traditional ways. You can pay in different ways and customise your food in different ways. I think we’re trying to add more choice and variety.”