The BBC had hoped to turn its Adobe AIR application into a standard for UK broadcasters, offering them an alternative, non-US-owned way to bring their UK content to viewers online.
The BBC had proposed that ITV, Channel 4 and Channel 5 should have access to iPlayer to enable them to compete with other TV companies when the UK switches over to digital TV in 2012.
The Trust said it "supported the principle of sharing the iPlayer more widely" but said the plans would not deliver value to licence-fee payers.
Diane Coyle, BBC Trustee and Chair of the Trust’s Strategic Approvals Committee said: "The iPlayer is a success, and we believe that access to its technology could be useful to other broadcasters. The Trust supports the BBC’s aim of sharing the benefits of the iPlayer.
"When assessing the proposals submitted by the BBC Executive, the Trust weighed up a number of factors. These included their strategic significance, their impact on other BBC activities, the potential competitive impact, and their overall value to licence fee payers.
"We concluded that the open iPlayer plans in their proposed form, combining both commercial and public service elements, were too complicated. We were not convinced that there was enough potential value to licence fee payers in the public service part of the proposal, and we have therefore rejected the BBC Executive’s proposals for an open iPlayer federation.
"We will look again at future public service models for the online delivery of programming as part of the strategic review now in progress. In the meantime, the Trust is open to considering an alternative proposal for the licensing of the iPlayer technology to third parties if that can be done on a simple, fair and commercial basis."
As a response, Channel 4 has announced it will be offering full-length shows through Google’s YouTube.