The backlash against Facebook’s “free mobile data” Internet.org scheme has spread across the globe.

A total of 67 digital rights groups – including i Freedom Uganda, Ecuador’s Usuarios Digitales and Indonesia’s ICT Watch – have signed a letter to Facebook’s founder, Mark Zuckerberg, stating concerns about the initiative.

They say the project threatens freedom of expression, privacy and the principle of net neutrality.

Facebook continues to defend its offer.

“We are convinced that as more and more people gain access to the internet, they will see the benefits and want to use even more services,” a spokesman told the BBC.

“We believe this so strongly that we have worked with operators to offer basic services to people at no charge, convinced that new users will quickly want to move beyond basic services and pay for more diverse, valuable services.”

Internet.org allows subscribers of partner mobile networks to use a limited number of online services without having to pay to make use of the data involved.

They include Wikipedia, the Facts for Life health site run by the United Nations Children’s Fund, BBC News, Facebook, Accuweather and a selection of local news and sports results providers.

To access the facility, people must use special Android apps, Internet.org’s website, Facebook’s own Android app or the Opera Mini browser.

The web pages provided must be basic to minimise data use – high resolution photos, videos and voice chat facilities are not permitted.

Network operators participate because they believe users will pay for wider internet access once they have had a chance to try out the free content on offer.

Since 2014, the project has launched in Zambia, India, Colombia, Guatemala, Tanzania, Kenya, Ghana, the Philippines, Indonesia and Malawi.

Facebook says more than nine million people have used the scheme to date.