Unless spectrum is set aside at affordable rates for public service broadcasters such as the BBC, ITV and Channel 4, Sky could end up being the UK's only high-definition TV platform, according to Grade.
Public service broadcasters would be unable to compete with the deep pockets of subscription services like Sky and Virgin Media, resulting in an "HD divide" between pay-TV and Freeview viewers.
The group believes that an auction will leave Freeview viewers, most of whom are expected to have HD sets by 2010, unable to watch high-definition TV.
The campaign wants part of the spectrum ring-fenced for Freeview, but Ofcom claims that it does not have the power to allocate spectrum for specific uses.
However, Grade claimed that there is a get-out clause that allows the government to determine the allocation of spectrum.
Culture and Media Secretary Tessa Jowell has faced pressure from a cross-party group of MPs to overrule Ofcom's auction plan and allow at least a third of spectrum to go to public service broadcasters.
"We have a long and honourable tradition in the UK of ensuring that everyone has access to the same quality of broadcasting," said Grade.
"That is precisely what digital switchover seeks to achieve. Closing the door to HD on Freeview would undermine that principle."
Mobile TV, standard-definition digital TV and DAB digital radio are among the platforms competing with HDTV for spectrum space, which will be freed up as analogue signals are switched off between 2008 and 2012.
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