Internet.org, Facebook’s initiative to expand internet access to the “next billion” people across the developing world, has come under attack from 67 digital rights groups worldwide for not doing enough to promote net neutrality alongside expanding access.

The signatories, which include America’s 18MillionRising.org and Europe’s Free Press Unlimited as well as India’s IT for Change and Nigeria’s Paradigm Initiative, argue that Facebook is “building a walled garden in which the world’s poorest people will only be able to access a limited set of insecure websites and survives”.

“Further, we are deeply concerned that Internet.org has been misleadingly marketed as providing access to the full internet, when in fact it only provides access to a limited number of internet-connected services that are approved by Facebook and local ISPs,” it continues.

The letter, ironically published on Facebook and addressed to Mark Zuckerberg personally, addresses eight separate areas where the signatories perceive Internet.org’s offering as lacking.

Top of the list is net neutrality, the term for treating internet connections the same regardless of what content is being sent over them. The signatories argue that although Facebook says it supports net neutrality, it is “improperly defining” the term, and instead favouring certain services over others. Key to their critique is that Internet.org encourages “zero rating” websites, working with site owners to make accessing their website free of charge on metered internet connections. Facebook Zero and Wikipedia Zero are two such projects.

But the signatories also argue that Internet.org poses problems for privacy and security online. “Facebook’s privacy policy does not provide adequate protections for new internet users, some of whom may not understand how their data will be used, or may not be able to properly give consent for certain practices.” On top of that, the current implementation of Internet.org prohibits the use of basic web encryption such as SSL or TLS. “This inherently puts users at risk, because their web traffic will be vulnerable to malicious attacks and government eavesdropping.”

As a result, they say that Internet.org runs the risk of creating “a two-tier internet”.

“The economic boom and revolution in connectivity that the internet created in developed countries needs to be shared equally with the next three billion people. Internet.org’s model – giving users a taste of connectivity before prompting them to purchase pricey data plans – fails to acknowledge the economic reality for millions of people who can’t afford those plans.

“These new users could get stuck on a separate and unequal path to Internet connectivity, which will serve to widen – not narrow – the digital divide.”

In April, Mark Zuckerberg directly addressed criticism of Internet.org from India, where many of the same points had been made. “Some people have criticised the concept of zero-rating that allows Internet.org to deliver free basic internet services, saying that offering some services for free goes against the spirit of net neutrality. I strongly disagree with this,” Zuckerberg wrote.

“To give more people access to the internet, it is useful to offer some service for free. If someone can’t afford to pay for connectivity, it is always better to have some access than none at all.”

A Facebook spokesperson said: “We and our critics share a common vision of helping more people gain access to the broadest possible range of experiences and services on the internet. 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. 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.”