A birth control app under investigation in the UK for causing unwanted pregnancies has been given the green light to be advertised as a contraceptive by American drug regulators.

The Food and Drug Agency said the app, called Natural Cycles, was the first of its kind to be marketed in the US.

Natural Cycles uses an algorithm that calculates the days of the month a woman is likely to be fertile. It takes daily body temperature readings and menstrual cycle information into account, building on a traditional method called fertility awareness.

The app costs around £7 per month and it currently has more than 700,000 active users. It claims to be the only certified birth control app in Europe. However, it is not regulated by the British Medicines and Healthcare Product Regulatory Agency.

The product's use has not been without controversy. The British Advertising Standards Agency is currently investigating Natural Cycles following its Facebook advertising campaign in which it described its service as “highly accurate”.