The BBC's governing body has rejected a series of complaints about a ban on non-religious contributors to the Radio 4 Today programme's Thought for the Day.
The BBC Trust said yesterday that only allowing religious contributors on the slot did not breach editorial guidelines on impartiality.
But it ruled that Thought for the Day must comply with requirements of "due impartiality" and that any future complaints on broadcasts during the slot would be judged on a "case-by-case basis".
The ruling comes after 11 complaints about Thought for the Day and one complaint about BBC editorial policy on non-religious programming. Complainants objected to a policy of not inviting atheists, secularists or humanists to contribute as speakers.
A statement from the trust said it considered that it was a matter for the BBC executive board as to whether the remit of Thought for the Day should remain the same or be changed in the future.
Terry Sanderson, president of the National Secular Society, which was one of the complainants, said it was "very disappointed" by the decision and would continue to campaign to "open it up" to other groups. "Every edition of Thought for the Day is a rebuke to those many people in our society who do not have religious beliefs," he said.
"It says to them that their 'thoughts' are not worth hearing and that somehow religious opinions are more worthy of a special, unchallengeable platform.
"This is so blatant an abuse of religious privilege that we cannot simply let it pass. We will be looking at other ways of challenging this unjustifiable slot."
A Church of England spokesman said: "We are glad that the BBC Trust has protected a unique slot in Radio 4's schedule where religious views from across the faith communities of the UK can be expressed openly.
"Thought for the Day is highly valued by people of all faiths and none as a distinctive slot that, if diluted, would become nothing more than just another comment slot."
Andrew Copson, the British Humanist Association's director of education and public affairs, said the trust's ruling was "a shame". He added: "We can see no good reason why humanists are barred from making their contribution."
A Radio 4 spokesman said: "We note the trust's findings with interest. There are currently no plans to make any changes to Thought for the Day."