By Dominic Casciani
BBC News home afairs
A group saying it represents large numbers of "ex-Muslims" is urging policy-makers to ignore the faith.
Islam: Group argues many Muslims scared to leave the faith
Campaigner Maryam Namazie said 25 founding members were being named at the body's Westminster launch, representing people scared to speak.
The Council of ex-Muslims believes it represents the views of a majority of secular-minded Muslims in Europe.
The Muslim Council of Britain, the largest umbrella body in the UK, declined to comment on the launch.
Ms Namazie said the new organisation would be a branch of a growing network of secular "ex-Muslims" who oppose the interference of religion in public life.
It is supported by the British Humanist Association and National Secular Society and is associated with groups in other European countries, principally Germany.
Iranian-born Ms Namazie is a human rights activist whose family fled the country during the 1980 Islamic Revolution. She has frequently challenged religious thinkers for the way she says they try to control the lives of individuals, particularly women.
The new group would be an alternative voice to bodies like the Muslim Council of Britain, she told the BBC, saying many people who disagreed with the opinions of religious leaders were scared of speaking out.
Ms Namazie urged governments to stop dealing with Islamic organisations that were pushing their values on other people and limiting free speech.
"We are taking a stand for reason, universal rights and values, and secularism. We are quite certain we represent a majority in Europe and a vast secular and humanist protest movement in countries like Iran," she said.
"People can have their beliefs but they must be kept in the private sphere."
"We don't think people should be pigeonholed as Muslims or deemed to be represented by regressive organisations like the MCB," she said.
Ms Namazie added: "Those of us who have come forward with our names and photographs represent countless others who are unable or unwilling to do so because of the threats faced by those considered 'apostates' - punishable by death in countries under Islamic law."
In some parts of the Islamic world, apostasy is punishable by death - Italy gave asylum in 2006 to an Afghan man who said he would have been killed for converting to Christianity.
Other thinkers, including European scholars, argue that the call for punishment is too literal and ignores a key section of the Koran that says people cannot be compelled to religion.
The UK government and local authorities have policies of dealing directly with faith bodies, including providing funding. However, ministers are debating whether to change the rules to ensure money goes on boosting cohesion between different peoples.