The past pupils of fee-paying schools in Dublin are joining forces to campaign against what they describe as “unjust State interference” in their school’s admissions policies.

Former students of Blackrock College and Belvedere have urged fellow alumni to campaign against provisions in the Admissions Bill which would prevent schools from reserving places for the sons of past pupils.

In a letter to members of the Blackrock College Union, described as fellow “Rockmen”, union president and barrister Shane Murphy SC says “we have engaged in dialogue with representatives of other past pupils organisations” and “we would encourage you to become actively involved to ensure that this proposal is defeated.”

“The admissions policy (at Blackrock) has engendered a positive community spirit through many generations of ‘Rockmen’ and has never before been so unjustly challenged by the State,” he writes.

“Anything that potentially threatens the tradition where brothers and sons of past students can follow in the footsteps of their brothers and fathers through Blackrock College is a threat to that which many of us hold so dear.”

The letter, sent last Wednesday, follows a similar plea by the Belvedere past pupils union to its Jesuit-educated alumni.

“As a past pupils union, we believe this to be the single biggest threat to the future of Belvedere college and its survivability for the future. This isn’t just a threat to us, but a threat to all schools in the country who rely on the goodwill generosity and general buy-in from past pupils,” writes the union’s president Cillian Murphy.

“If you had aspirations of sending your Son or Grandson to Belvedere, think again. Despite your generosity, commitment and bond to Belvedere College, this Government is firmly intent on denying this right.”

The letter continues: “If you have a Son who is due to leave the College in 2016 and your second Son was due to join the following year, he gets no priority as his sibling has already left, and if you’re a teacher who has committed a lifetime to Belvedere, this Government doesn’t care.”

Minister for Education Jan O’Sullivan plans to introduce new legislation in the coming months to combat discrimination in school admissions.

Only a draft Admissions Bill has been published to date but it includes proposals to end waiting lists for admissions, as well as the practice of charging application fees. It says, however, schools would be allowed to prioritise places for an applicant who is a sibling of an existing or former student.

The legislation is aimed at ensuring all schools in receipt of state funds are inclusive of different social backgrounds and educational needs.

In his letter to Blackrock alumni, Mr Murphy says: “This legislative proposal is potentially one of the greatest challenges facing the college and the union in recent years.”

The school union has already been in contact with similar bodies in other schools, he writes, and “we would encourage you to become actively involved to ensure that this proposal is defeated.”

Warning of the “threat” to the school’s ethos, he claims: “The Government is proposing that the admission policies of all secondary schools should be standardised and controlled by the State.”

Past pupils of the all-boys, fee-paying school include former minister for education Ruairi Quinn who initiated the bill; broadcaster Ryan Tubridy; rugby star Brian O’Driscoll; financier Derek Quinlan as well as several High Court judges.