Birmingham City Council has been told it could face legal action for blocking its workers from looking at atheist websites.
Software used by the council stops employees from accessing sites about atheist beliefs, but sites about conventional religions are readily available.
The National Secular Society said that such filtering is discriminatory and it may take legal action against the council unless it changes its policy.
The society's president Terry Sanderson described the blocking of atheistic websites as "slightly deranged".
He said: "It's an open-and-shut case of discrimination as far as we're concerned. If you can access religious sites you should be able to access sites for non-religious people.
"We've taken legal advice from our barrister and she tells us quite clearly that there is no justification for it."
Mr Sanderson said that either atheist websites should be fully accessible or conventional religious websites should also be blocked.
He added: "If the council won't change it then we would have to look at legal action."
A spokeswoman for Birmingham City Council would not comment on the threat of legal action