More people in Northern Ireland now identify as Catholic than Protestant for the first time in the history of the jurisdiction, new census figures reveal.

The data has led to calls for a referendum for voters to decide whether to remain part of Britain or join with the rest of Ireland and form a new country.

It comes 101 years after Northern Ireland was established in the six northeastern counties on the island of Ireland, remaining part of Britain when the 26 southern counties won independence from British rule.

The founders of Northern Ireland drew the boundaries of the state along lines that they hoped would guarantee a permanent Protestant majority. Traditionally, Protestants have supported being part of Britain, whereas the Catholic community has traditionally supported unity with the rest of the island to form a single independent Ireland.

The first prime minister of Northern Ireland, Sir James Craig, famously addressed the legislature describing it as a "Protestant parliament for a Protestant people," and the Catholic minority complained of discrimination in terms of jobs, housing and voting rights.

The proportion of the resident population that is either Catholic or brought up Catholic is 45.7%, compared to 43.5% Protestant.

The previous census, in 2011, found that 45.1% of the population were Catholic or brought up Catholic. It found 48.4% were from a Protestant or other Christian background.

Census figures detailing the religious makeup of Northern Ireland were published by the Northern Ireland Statistics and Research Agency Sept. 22.

Deirdre Heenan, professor of social policy at the University of Ulster, described the first census showing a Catholic majority as "monumental."

"We have to look back at the history of the state, how it was created, the decades of discrimination (against Catholics) and, really, what we then need to do is have a look at where we are at the moment and what we want for our future," Heenan told Catholic News Service.

Under the terms of the 1998 Good Friday Agreement, which brought an end to 30 years of sectarian conflict that killed more than 3,500 people, the constitutional status of Northern Ireland can be changed only with the consent of its population.

The British government is obliged to call a referendum on Irish unity if it believes there has been a shift in public opinion that would indicate a majority in favor of constitutional change.

John Finucane, a lawmaker with the Sinn Féin party, which campaigns for Irish unity, described the census results as "another clear indication that historic change is happening across this island and of the diversity of society which enriches us all."

"There is no doubt change is underway and irreversible. How that change is shaped moving forward requires maturity to take the challenges which face our society," he said.

"We can all be part of shaping a better future -- (a) new constitutional future and a new Ireland," he said.

The growth of the Catholic population has continued steadily since the first Northern Ireland census in 1926, dipping only during the early years of the 1968-1998 civil conflict known as "the Troubles," when many Catholics emigrated.

In 1926, Protestants made up at 66.3% of the population while Catholics accounted for 33.5%.

In the current census, close to 20% of the population said they have no religion, up from approximately 10% reported in the previous census.

The majority of those listing no religion on the census come from a Protestant background, according to the figures.