Dr Maurice Hayes, the only Northern Ireland Catholic on the Patten Commission on Policing, has recommended that Catholics join the new force.
The outspoken Co Tyrone clergyman, Mgr Denis Faul, has also urged Catholics to join the new Police Service of Northern Ireland in large numbers as the only real way to change the ethos of the force.
Dr Hayes, a former ombudsman for Northern Ireland and a member of the Seanad, said yesterday that nationalist parties would have to decide whether they would "settle for 90 per cent of something".
He said yesterday: "I would be more or less in line with Mgr Faul in calling for Catholics to join. Basically the nationalist parties have to decide whether they want 90 per cent of something or 100 per cent of nothing.
Mgr Faul said that while neither the Patten report nor its implementation was in any way perfect, people in Northern Ireland did not have the time to wait for more effective legislation. Instead they should join up now and try to create the service they wanted by influencing it from the inside, he told The Irish Times.
"I am not satisfied with many aspects of Patten itself, nor with its implementation. But the safety of the people is the supreme law. If young Catholics and Protestants form a conscious desire to perform their civic duty, that should be respected. Let young people make up their own minds."
Mgr Faul made his comments as republicans reiterated their determination not to nominate members to the new Policing Board or to encourage young nationalists to join the service after the Northern Ireland Police Bill received royal assent on Thursday.
He said he was particularly concerned about Mr Patten's recommendation to increase the part-time reserve from the present 1,300 officers to 2,500.
Nonetheless he urged Catholics to join the police and "try to ensure that the rights named in the European Convention on Human Rights are available to all citizens.
"I hope that public opinion against private armed groups and bully-boy assaults will enable the police to provide security, privacy and fair play to all citizens. A proper promulgation of the convention could help to form the requisite public opinion," he concluded.
The Sinn Fein vice-president, Mr Pat Doherty, said his party had no intention of taking its seats on the Policing Board and would not advise nationalists to join up without the full implementation of Patten.