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Northern Ireland has more Catholics than Protestants for the first time, census results showed on Thursday, a historic shift that some see as likely to help drive support for the region to split from Britain and join a united Ireland.

The shift comes a century after the Northern Ireland state was established with the aim of maintaining a pro-British, Protestant "unionist" majority as a counterweight to the newly independent, predominantly Catholic, Irish state to the south.

At that time, the population split was roughly two-thirds Protestant to one-third Catholic.

Data from the 2021 census released on Thursday showed 45.7% of respondents now identified as Catholic or were brought up Catholic, compared with 43.5% identifying as Protestants.

A decade ago the previous census showed Protestants outnumbered Catholics by 48% to 45%, after falling below the 50% mark for the first time.

Northern Ireland's sectarian divisions can be traced back to the 17th Century, when Protestant settlers from Scotland and England were "planted" in the northeastern part of the island to bolster the authority of the English Crown.

Demographers have long predicted that Catholics, who tend to be younger and have higher birth rates, could become a majority of voters within a generation.

A regional election victory by Irish nationalists Sinn Fein in May, which saw them secure the largest number of seats for the first time, shocked many unionists.

Colum Eastwood, the leader of the smaller, moderate Irish nationalist party the SDLP, said on Thursday the shift revealed in the latest census numbers was "a seminal moment in the history of modern Ireland" that should not be downplayed.

But while Catholics tend to vote for Irish nationalist parties and support the cause of a united Ireland, an increase in Catholic population does not automatically increase support for either.

A significant minority of Catholic and Protestant voters support the cross-community Alliance Party, which doubled its number of seats in the May election.

Polls also show that not all Catholics and Protestants support the predominant views on a united Ireland in their communities.

Another question in the census found that 43% identified as British, down from 48% 10 years ago, with 33% seeing themselves as Irish, up from 28%. Another 32% said they were Northern Irish. People could select more than one national identity.

Before a 1998 peace deal, more than 3,000 died during three decades of fighting between mainly Catholic Irish nationalist militants seeking a united Ireland they believed would guarantee their rights, mainly Protestant pro-British loyalists and the British Army.