It is an unholy legal minefield involving two pieces of legislation, Scotland's biggest council, the Catholic Church and an atheist.

When David McNab successfully argued that his human rights had been breached after he was prevented from applying for promotion at a Catholic school, the decision was seen by some as the end of the Church's age-old right to veto staff.

Mr McNab, then a maths teacher at St Paul's High in Glasgow, went to an employment tribunal after being told he would not be considered for the post of principal teacher of pastoral care at the school because he had not been approved by the Church.

The tribunal ruled in March last year that the post in question did not require approval, and therefore found in Mr McNab's favour.

Soon after winning his case, Mr McNab said he hoped the decision would see an end to the right of the Church to block teachers from employment or promotion on the grounds of their "religious belief and character".

However, in the weeks following the case, it became clear that other aspects of the ruling might have an entirely different impact on the employment of teachers in denominational schools.

The tribunal decided that an informal agreement drawn up between the former Strathclyde region and the Church in 1991, under which non-Catholics were excluded from certain key posts in denominational schools, such as headteacher, assistant head, and teacher of religious education, biology, or guidance, was unlawful.

The ruling stated that the 1980 Education (Scotland) Act "does not permit the Catholic Church to reserve certain posts for teachers who are Roman Catholics".

Instead, the ruling reaffirmed the Church's right to approve all teachers in its schools enshrined in the 1980 act, but never fully observed, with many councils choosing to continue to reserve specific posts after local government reorganisation.

The Scottish Catholic Education Commission (SCES) seized on the broader implications of the findings immediately, and put forward a set of proposals which would enable councils to abide by the 1980 act.

Officials from the commission said that all teachers applying for a job should provide the name of a referee to testify to their commitment to Catholic schools. In addition, teachers should agree to promote the values set out in the Charter for Catholic Schools in Scotland, which states that teachers should have a commitment to uphold the moral teaching, faith tradition, and sacramental life of the Church.

For the Church, the issue is not one of excluding non-Catholics, but of ensuring that every teacher who works in a denominational school understands the distinctive aims and values of Catholic education.

However, the suggestion that all staff would need approval was met with horror by teaching unions, who said that no qualified teacher who was registered with the General Teaching Council of Scotland and had passed a police disclosure check should be denied a job on "religious or moral grounds".

There was further concern over how the 1980 act tallied with 2003 employment regulations, which deem it unlawful for an employer to discriminate against anyone on the grounds of religion or belief.

It was into this minefield that Glasgow City Council's legal experts stepped, with a view to resolving the issue to the satisfaction of all.

A confidential document seen by The Herald shows that the council is now recommending the SCES proposals are adopted, but they also acknowledge the inherent difficulties that still exist.

"There is a tension between the council's management of denominational schools as education authority and the prohibition against discriminating in employment on the grounds of religion or belief," the document states. "It may be that this tension can only be resolved by the Scottish Executive or a superior court."

Mr McNab, who is now working in a special needs school, agrees. He sees potential legal challenges on two fronts: "On the one hand, it is likely that those who are not given approval at all will challenge that decision through the courts," he said.

"Even more interesting will be a situation where someone who already has approval, but is not a Catholic, goes for a critical post such as headteacher and is refused.

"This ruling opens the door for non-Catholics to apply for promoted posts for the first time and, if they are prevented from doing so, then we are likely to see the matter brought swiftly back to the courts."

The legislation

Education (Scotland) Act 1980
A teacher appointed to any post in a denominational school requires to be approved as regards his or her religious belief and character by the Church or other denominational body.

Employment Equality (Religion or Belief) Regulations 2003
It is unlawful for an employer to discriminate against anyone on the grounds of religion or belief. The only exception is where there is a general occupational requirement for a post to be held by a person of a specific religion or belief.