An advert on Facebook for an app that provides a natural alternative to contraception has been banned by the UK's Advertising Standards Authority.
Claims that it was "highly accurate" and "provided a clinically tested alternative to other birth control methods" were found to be misleading.
The Swedish firm behind the Natural Cycles app was warned "not to exaggerate" its efficacy.
In response, the firm said it respected the outcome of the investigation.
It told the BBC that it removed the ad, which ran for approximately four weeks in mid-2017, as soon as it was notified of the complaint.
"We are committed to being open and transparent in our communications to ensure our message is clear and provides women with the information they need to determine if Natural Cycles is right for them. As part of these efforts, every advertisement undergoes a strict approval process," the firm said in a statement.
"Natural Cycles has been independently evaluated and cleared by regulators in Europe and the US based on clinical evidence demonstrating its effectiveness as a method of contraception."
The ASA said that the Facebook ad must not appear again in its current form.
Natural Cycles requires women to take their temperatures every day using a basal body thermometer and to enter the reading into the app, which also tracks a user's menstrual cycle.
The app uses an algorithm to determine a woman's daily fertility based on changes in basal body temperature.
Basal thermometers are able to detect a minor rise in temperature around the time of ovulation. Women will see a "use protection" warning appear on the app during their fertile days.
Launched in 2014, the app now has more than 300,000 users who pay a monthly or annual fee for the service.
It was invented by Swedish nuclear physicist Elina Berglund Scherwitzl and her husband.
It has previously been approved for use as a medical device by German inspection and certification organisation Tuv Sud, which means it can be used across the EU.
And it recently won approval from the US Food and Drug Administration, which described it as an effective method of contraception if "used carefully and correctly".
At the time, Terri Cornelison, assistant director for the health of women at the FDA's Center for Devices and Radiological Health, said: "Women should know that no form of contraception works perfectly, so an unplanned pregnancy could still result from correct usage of this device."
In considering the complaint, which was lodged by three people, the ASA took expert advice and reviewed three published papers based on the accumulated data obtained from the app.
It found that there was a distinction between typical use of the app and the perfect-use scenario. It felt that the figures presented to users were based on the perfect user rather than typical user and, for the latter, the system could not be described as highly accurate.
It also found that presenting the statement "highly accurate" alongside the claim "clinically tested alternative to birth control methods" gave the impression "that the app was a precise and reliable method of preventing pregnancies which could be used in place of other established birth control methods".
Natural Cycles told the ASA that the claim "clinically tested alternative to birth control methods" was a quote from the news site Business Insider.
But, it added, the claims were backed by scientific evidence, including clinical trials.
Separately the app is being investigated by Sweden's Medical Products Agency (MPA) which told the BBC it has received approximately 60 complaints relating to unwanted pregnancies as a result of using the app.
It is due to publish its conclusions next week.
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