The city council said the system helped managers monitor staff web access
A city council has blocked its staff from looking at websites about atheism.
Lawyers at the National Secular Society said the move by Birmingham City Council was "discriminatory" and they would consider legal action.
The rules also ban sites that promote witchcraft, the paranormal, sexual deviancy and criminal activity.
The city council declined to comment on the possible legal action, but said the new system helped make it easier for managers to monitor staff web access.
'Very strong case'
The authority's Bluecoat WebFilter computer system allows staff to look at websites relating to Christianity, Islam, Hinduism and other religions but blocks sites to do with "witchcraft or Satanism" and "occult practices, atheistic views, voodoo rituals or any other form of mysticism".
Under the Employment Equality (Religion or Belief) Regulations 2003, it is unlawful to discriminate against workers because of their religion or belief, which includes atheism.
National Secular Society president Terry Sanderson said the city council's rules also discriminated against people who practise witchcraft, which is also classed as a legitimate belief.
He said the society would initially contact the council and ask for the policy to be changed, and otherwise pursue legal action.
He said he believed he would have a "very strong case".
Mr Sanderson said: "It is discriminatory not only against atheists but they also are banning access to sites to do with witchcraft.
"Witchcraft these days is called Wicca, which is an actual legitimate and recognised religion.
"We feel very strongly that people who don't believe should not be denied the access that people who do believe have got."
He added that some opinion polls said that up to 25% of the UK population now considered themselves atheist.
A city council statement said the authority had a "long-standing internet usage policy for staff".
It added: "We are currently implementing new internet monitoring software to make the control of internet access easier to manage.
"The aim of this is to provide greater control for individual line managers to monitor internet usage, and for departments, such as trading standards and child protection, to gain access, if needed, to certain sites for business reasons."