The latest comments by Sky are part of a long-held attack on the BBC, Five, ITV and BT led venture, which aims to set a new technical standard for IP-enabled TV devices.
Chief among concerns for the satellite platform holder is that public money will be used to dominate the nascent video on-demand market and that non public service broadcasters will be locked out of the process.
In response, the BBC has published repeated assurances about the open structure of a Canvas platform, and further moved to change the initial governance structure of the project so that non-PSBs can now take an equal share in the venture.
However, Sky has continued with its campaign to persuade the BBC Trust to prevent the project from going ahead.
"Given its unique and privileged position in receipt of substantial and guaranteed public funding, the current proposals remain inconsistent with the BBC's obligations to adopt the least intrusive and most proportionate means of fulfilling its core public service purpose," Sky said in a statement reported in The Guardian.
"Sky does not believe that Canvas should be allowed to proceed, or at least, not with the BBC, and the licence fee, playing an active role. The proposals for membership of the joint venture remain exclusionary.
"The new proposals pay lip service to the idea of an open joint venture and the governance changes will make little difference in practice to the current Canvas members' ability to pick and choose their partners in the future. The admission criteria to be applied by the Canvas board are subjective and therefore open to arbitrary interpretation."
Sky also claimed that it was "fanciful" to suggest Canvas would be an open platform when one of its founding partners BT runs the pay TV platform BT Vision and intends to replace that service with Canvas.
The firm further said that the estimated £20 million cost to become a partner in the project is "set at a level that will exclude many, if not all, new members". It was recently revealed that Canvas will cost £115m over its first four years, including a marketing budget of £48.4m.
Sky said that the outlined investment in promoting Canvas is nearly double the amount spent backing Freeview, and is very high considering the "clear overlap between the two services and likely cross-promotional marketing".
The firm added: "More particularly, the budget is disproportionately high when compared to any other start-up and cannot be said to represent 'value for money' for the licence fee payer or be proportionate when considering the application of state aid law."