The Catholic church is taking the government’s schools admissions watchdog to the high court to protect the rights of priests to determine whether pupils are eligible for a place on the basis of their faith.

The schools adjudicator ruled earlier this month that a new policy across all Catholic schools under which priests certified on a pupil-by-pupil basis whether they were from a “practising [Catholic] family” was unfair.

The ruling followed a complaint from a parent hoping to secure a place at the St Richard Reynolds Catholic college, in Twickenham, west London. The parent, and Surrey county council, complained that “the arrangements do not define what form or frequency of religious practice is required for a priest to do this.

This academic year, the Catholic Education Service, which decides policy for the 2,300 Catholic schools in England and Wales, introduced a certificate of Catholic practice (CCP) to ensure all its schools followed the same admissions policy. The previous ad hoc arrangements had been found to be open to inconsistent application and abuse, with parents without any genuine faith seeking to game the system to secure a school place.

Under the current law, faith schools can select on the basis of religious faith when they are oversubscribed. The government is proposing to lift a cap of 50% on the proportion of pupils selected by faith in free schools and academies. Voluntary-aided faith schools can select up to 100% of pupils on the basis of faith.

Catholic schools outperform the national average English and maths scores by 6% at Sats assessments (age seven-14); at GCSE, they outperform the national average by 5%.

However, the Office of the Schools Adjudicator decreed that the basis on which priests made their decisions was not transparent. “There is no easy way for any parent to know in advance that they will be able to fulfil the oversubscription criteria under which such priority is afforded,” the ruling said.

St Richard Reynolds Catholic college, Twickenham.

“Even those parents who know, because they are ‘practising Catholics’, that canon law requires that they attend mass on a weekly basis from at least seven years of age (to use the diocese’s words) and who attend mass in accordance with those requirements cannot be certain that they will be ‘granted’ (the term used in a letter to me from the diocese concerning one of the other schools) a CCP.

“The arrangements do not make any reference to this being the case, and a meeting with the parish priest is still required at least in some cases, (although in all cases according to that same letter) before a CCP is issued.”

The ruling adds: “No parent will know for certain whether or not they will be given a CCP. Such a parent may have attended mass every week for many years, or they may be a recent convert to Catholicism or a person with extenuating circumstances that have prevented such attendance, but on reading the school’s admission arrangements they would both be in the same position of not knowing whether they would be given priority on the grounds that they are a practising Catholic should they apply for a place at either school.”

A spokesman for the diocese of Westminster, within which St Richard Reynolds Catholic college falls, said: “The legality of this determination is now being challenged in the high court.”

A spokesman for the Catholic Education Service said that the religious community accepted that priests were the right adjudicators: “This academic year in order to improve the consistency across the country, there has been a move from diocesan priests’ references to a national system of a certificate of Catholic practice. This ensures that the measures used to determine Catholic practice are the same across diocesan boundaries.

“In a few parts of the country there are insufficient places for all Catholic children and therefore some schools will decide to use a criterion of practising Catholic in their admission arrangements, and therefore use the CCP,” she said.

“The central issue in the determinations is who should decide whether a pupil is a practising Catholic. Within the Catholic community, it is accepted that priests are the correct authority to identify any practising Catholics.”

Rabbi Dr Jonathan Romain, chair of the Accord Coalition, representing campaigners against faith schools, welcomed the development: “Selecting pupils on religious grounds is inherently more complex and open to abuse, gaming and error than other forms of selection. Non-religiously selective faith schools generally avoid these problems,” he said. “These are welcome rulings, but there is a bigger problem around religious selection, and how it leads to confusion, unfairness and systemic disadvantage to some children, especially from certain ethnic and deprived backgrounds.”