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Several leading UK internet service providers have refused to sign a code of conduct designed to guarantee "full and open access" to the net.

Ten ISPs including BT, O2 and Talktalk backed the agreement promising not to restrict or block content unless there was a reason to deploy "reasonable traffic management practices".

But Virgin Media said the principles set out were too vague while Vodafone said the code was "impractical".

Everything Everywhere also opted out.

The Open Internet Code of Practice builds on an earlier traffic management agreement - which the three hold-outs did agree to - adding three new commitments:

Exceptions to the rule include sites or services blocked by a court order; the need to manage congestion on the network if too many people are using data-hungry services at once; the imposition of data caps that are part of a user's contract; and the use of parental blocks deployed to keep children safe.

If breaches of the code occur they will be considered by a forum including the ISPs, the communications regulator Ofcom and media companies, known as the Broadband Stakeholder Group.

Net neutrality campaigners have long warned of the risk of a two-tiered system under which ISPs could charge premium rates for full internet access, or act to ensure their own video content was sent glitch-free while throttling material sent by other catch-up TV services.

The Netherlands became the first European country to pass a law forcing ISPs to guarantee equal access in May. However, no such right exists in the UK, although <link> <caption>Ofcom has warned</caption> <url href="http://stakeholders.ofcom.org.uk/binaries/consultations/net-neutrality/statement/statement.pdf" platform="highweb"/> </link> it might intervene if it saw a problem developing.

Ed Vaizey MP, the Minister for Culture, welcomed the new code.

"The internet has been built on openness and low barriers to entry, and this agreement will ensure that continues," he said.

Be, BSkyB, Kcom, Giffgaff, Plusnet, Tesco Mobile and Three also signed up to the plan, while Microsoft and the ´óÏó´«Ã½ were among those to praise its creation.

The businesses which declined said they all supported the idea of an open internet, but had qualms about the code itself.

Virgin Media suggested the circumstances under which traffic management practices could be deployed needed to be defined more strictly.

"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 spokesman.

Vodafone said it objected to the agreement because of the restrictions it would impose on the way it marketed some of its subscription packages.

"These plans offer internet access to smartphone and dongle users, but under the code we would have been unable to use the phrase 'internet access' to describe many of the services enjoyed by customers," a spokesman explained.

Everything Everywhere - which runs the T-Mobile and Orange mobile networks - said it was simply 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 a spokeswoman.

The Broadband Stakeholder Group said it believed the code did have the "right approach", but "could not comment on the individual views of non-signatories".