Ten ISPs, including BT, O2 and Talk Talk, support the code, which has also been praised by Microsoft and the BBC, according to BBC news.
But the ISPs that have declined to sign up to the code said that while they support the idea of an open internet, they have qualms about the code itself.
Virgin Media said the principles set out were too vague. It said the circumstances under which traffic management practices could be deployed needed to be defined more strictly.
Vodafone said the code was "impractical" because of the restrictions it places on marketing packages. "These principles remain open to misinterpretation and potential exploitation so, while we welcome efforts to reach a broad consensus to address future potential issues, we will be seeking greater clarity before we consider signing," said a Vodafone spokesman.
Everything Everywhere, which runs the T-Mobile and Orange mobile networks, said it was not ready to join. "We believe it is too early to know how a code of this type will affect customers' internet experience, but it is something we will continually review," said an Everything Everywhere spokesman.
Open internet commitments
The Open Internet Code of Practice builds on an earlier traffic management agreement, adding commitments for ISPs to:
- Guarantee open and full access to the internet across their range of products;
- Refrain from marketing a subscription package as including "internet access" if certain kinds of legal content or services are barred;
- Refrain from targeting or degrading content or applications offered by a rival.
Exceptions to the rule include sites or services blocked by a court order, the need to manage congestion on the network, the imposition of data caps that are part of a user's contract, and the use of parental blocks to keep children safe.
Breaches of the code will be reviewed by a forum that includes the ISPs, the communications regulator Ofcom, and media companies making up the Broadband Stakeholder Group.
Management of internet traffic
Supporters of net neutrality have long campaigned against a two-tiered system in which ISPs can charge premium rates for full internet access, or act to prioritise their own video content over rivals.
In May, the Netherlands became the first European country to pass a law forcing ISPs to guarantee equal access, but no such law exists in the UK.
However, in a document outlining its approach to net neutrality, Ofcom outlines conditions under which it would intervene, such as when practices or policies result in specific harm to consumers.
Earlier this week, the European Commission launched a public consultation into net neutrality.
The consultation will be led by the vice-president for the digital agenda at the European Commission, Neelie Kroes, who is keen to establish the facts around management of internet traffic across Europe and ensure customers buying services are getting a fair deal.
The four areas the consultation will focus on will be internet traffic management, such as congestion management and privacy issues, transparency of adjustments or restrictions to internet performance, the ability for consumers to switch operators, and internet interconnection issues between network operators.