Humanists have welcomed a High Court ruling that the Education Secretary made "an error of law" when she left "non-religious world views" out of the new religious studies GCSE.
A judge declared that Nicky Morgan fell into error by asserting the GCSE, due to come into effect in the 2016 academic year, would "fulfil the entirety of the state's (religious education) duties".
Mr Justice Warby, sitting in London, said the assertion was in breach of the Government's duty to take care that the wider school curriculum ensured knowledge was conveyed "in a pluralistic manner", and information was given about atheist and other non-religious belief.
British Humanist Association (BHA) chief executive Andrew Copson described the "landmark" judgment as "a stunning victory".
Mr Copson said: 'We have made the case for many decades that the school curriculum on religions should include major non-religious world views such as humanism. It is great news that the court has now said the law is with us."
A Department for Education spokesman stressed the judgment had not declared the new GCSE itself unlawful, adding: "We will carefully consider the judgment before deciding on our next steps."
The spokesman said: "Our new RS GCSE ensures pupils understand the diversity of religious beliefs in Great Britain through the study of more than one religion, an important part of our drive to tackle segregation and ensure pupils are properly prepared for life in modern Britain.
"It is also designed to ensure pupils develop knowledge and understanding of both religious and non-religious beliefs."
The High Court ruling was a victory for three families, supported by the BHA, who complained the Government was failing to reflect in schools the pluralistic nature of the UK.
The families included one from Cumbria and one from Kent who cannot be identified. The third family is Kate Bielby, of Frome, Somerset, and her daughter Daisy.
The parents all hold non-religious beliefs, while the children in each family are all at academies and due to start on GCSEs in 2016-17.
Mrs Bielby said: "My daughter and I are delighted by today's decision and the clear statement that it makes in support of equality of religion and belief.
"It is long past time that the beliefs of the non-religious were treated on an equal footing with religions in the school curriculum.
"I am confident that whatever changes are introduced on the back of this judgment, Religious Studies will be a fairer, more inclusive subject, benefiting all children whatever their religious or non-religious background."
Changes to RS GCSE subject content were announced last February, leading to complaints over the priority given to religious views - in particular Buddhism, Christianity, Catholic Christianity, Hinduism, Islam, Judaism and Sikhism.
Earlier this year, the former Archbishop of Canterbury Rowan Williams was among 28 religious leaders who urged the Government to rethink plans to leave out humanism in the new qualification.
David Wolfe QC, for the families, told the judge at a recent hearing there was widespread concern "about the Secretary of State's failure to comply with her duty of neutrality and impartiality as between religious and other beliefs".
Lawyers for the Education Secretary said neither domestic law nor the European Convention on Human Rights require equal consideration to be given to religious and non-religious views in the curriculum.
They argued that, although some schools rely on the RS GCSE to discharge their duty to provide religious education at key stage 4 for 14 to 16-year-olds, provision has been made for non-religious beliefs to be studied and what is in a school's curriculum is a matter "for local determination" by individual school authorities.
The judge said: "It is not of itself unlawful to permit an RS GCSE to be created which is wholly devoted to the study of religion."
But he added the February announcement had included the "assertion" that the new GCSE "will fulfil the entirety of the state's (religious education) duties" and schools would interpret this to mean non-religious views need not be included in teaching.
"The assertion thus represents a breach of the duty to take care that information or knowledge included in the curriculum is conveyed in a pluralistic manner."
As a result, the Education Secretary "has made an error of law in her interpretation of the education statutes".
The judge made clear that his conclusions related to schools or academies which did not have a religious character.
Different provisions were made under the 1998 Schools Standards and Framework Act for schools that did have a religious character.
The judge said: "I have not found it necessary to address in this judgment the hypothetical question of whether the assertion might be true and lawful if and in so far as it relates to schools or academies with a religious character."