Thought for the Day briefly features each weekday on Radio 4's Today programme and it includes reflections from Christian and other faith perspectives.
Launching a new letter to lobby the corporation, the British Humanist Association said "so long as it exists, it is entirely unacceptable to exclude some speakers just because they are non-religious."
British Humanist Association supporters are being urged to sign the letter, which is being aimed at Chair of the BBC Trust Rona Fairhead.
It begins: "I am writing to express my frustration that Today's Thought for the Day slot continues to bar contributions from those with no faith.
"I do not believe that you need a 'faith perspective for an ethical view of the world.
"There is no good reason why non-religious people should not be able to hear views expressed that are representative of their opinions - especially as the number of licence-fee paying public who consider themselves to be humanists or of no faith continues to rise."
Describing Thought for the Day as a programme which "obtrudes itself on the general listener", the British Humanist Association has been a long-standing of the current format.
In a statement released in 2009, the organisation said: "The BBC is currently failing in its duty to reflect the diversity of beliefs of its audience and the wider population.
The new letter continues: "In ignoring humanists and atheists, Thought for the Day seems at odds with the BBC's ethos of 'fairness, balance and a voice for everyone in the country'.
"The current policy that expressly prevents non-religious speakers from contributing is plainly discriminatory.
"There is a wealth of humanist knowledge and thinking out there, ready to reflect on issues and people in the news, and such calls have been supported by its new editor Sarah Sands, by Today programme presenters like John Humphreys and Evan Davis, and even by the BBC's impartiality adviser during its last impartiality review.
"I therefore urge you to support any moves to change this policy in the future."
Premier has approached the BBC for a comment.