The British Humanist Association has joined parents and their supporters in a High Court battle against proposals for a new Roman Catholic secondary school.
The parents, who include Catholics, come from different backgrounds and say all new state schools in the London Borough of Richmond should have admissions policies that are religiously inclusive.
Today the humanist campaigners and the parents jointly asked a judge to overturn Richmond council's decision to offer the Catholic Diocese of Westminster a site in Twickenham to be used for two new schools - a primary and secondary.
They are seeking judicial review on the grounds that the decision was legally flawed.
Lawyers for Richmond argue the claim should be dismissed as the council did not misdirect itself or act unlawfully.
Andrew Copson, chief executive of the British Humanist Association (BHA), said the case reflected "a disturbing national pattern", in which religious groups were being given preferential treatment by local councils through "back-door proposals".
Mr Copson said outside London's High Court: "Victory here would hopefully set a precedent and level the playing field on which proposals to establish schools are treated equally, with the same level of scrutiny, whether religious or not."
The BHA is a charity campaigning on behalf of non-religious people seeking to live "ethical and fulfilling lives on the basis of reason".
The parents are being represented by the Richmond Inclusive Schools Campaign (RISC), which contends no child in the borough should be discriminated against because of family religious beliefs, or the lack of them.
RISC's leaders stress its supporters include both religious and non-religious people, including Catholics and humanists.
In 2011 over 3,300 local people supported a RISC petition to the council calling for all new borough schools to be inclusive.
But in July last year the council decided to buy land in Clifden Road, Twickenham, for £8.4 million to provide "school places".
It then invited the Catholic Diocese to submit proposals for two "voluntary-aided" Catholic schools and began a public consultation exercise.
In May this year, the council decided to lease the Clifden Road land to the Diocese for a peppercorn rent.
David Wolfe QC, representing both BHA and RISC, told the High Court the new primary school would, if oversubscribed, give priority to Catholic pupils when allocating all but 10 of its 30 places a year.
The secondary school would initially give priority to Catholics in all of its 150 places a year, although making exceptions for applicants coming from the 10 "community" places at the primary school.
After seven years, 93% of its places were expected to be allotted on the basis of religion.
Mr Wolfe said that, by contrast, if an academy/free school was set up under the 2006 Education and Inspections Act to meet the needs of Catholic families, children of non-Catholics "would be excluded from no more than half its places".
The QC argued the council went wrong in law because it was under a statutory obligation to seek proposals for an academy/free school rather than considering the Diocesan proposals for a voluntary-aided school.
He told Mr Justice Sales the council had conducted a flawed consultation exercise and sought to bypass its statutory duty, vitiating the decisions it had actually taken.
The BHA says the case is of general importance as it tests new laws and marks the first time the opening of a new school has been challenged on grounds of religious discrimination.
In February this year the Education and Inspections Act 2006 was amended.
Section 6A now requires English local authorities to set up proposals for establishing a free school/academy if they think a new school is needed in their area.
The Department for Education then holds a competition and adjudicates on which proposals should be approved.
All new free schools, if oversubscribed, have to have at least half their places open to all children regardless of faith.
Both BHA and RISC say Richmond Council repeatedly made it clear that it thought new Catholic schools were needed in the borough and should have taken the Section 6A route.
But the council agreed to approve voluntary-aided Catholic schools under a different section of the 2006 Act after deciding the schools were not needed "within the meaning of Section 6A".
Critics say the voluntary-aided route puts religious groups in a privileged position of not having to compete to open new schools.
Even if the courts take the view that the council did not believe the new schools were needed, it is argued the authority misled people, through its consultation exercise, into thinking that it did believe there was a need.
The hearing continues tomorrow.