The NHS will play a major role in deciding a border poll with support for the Union and a united Ireland neck-and-neck, according to a new survey.
A poll carried out by LucidTalk found that the health service would not influence how almost seven in 10 people voted in a referendum - 35% of people would vote for Irish unity and 34% would vote to remain in the UK regardless.
But over a quarter (26%) said that the NHS would make them more likely to support the Union.
Nationalists have acknowledged that the absence of a healthcare system free at the point of delivery in the Republic could be crucial in any referendum and must be addressed by those seeking constitutional change.
The NHS strengthens support for the Union particularly among older people, and Alliance and Green voters.
A total of 17% of Alliance voters said they'd definitely support Irish unity while 15% said the same for retaining the Union with the NHS playing no role in their decision.
But 54% of Alliance supporters and 52% of Greens said the healthcare system would make them more likely to vote to stay in the UK. More than a quarter of Greens (27%) would vote for Irish unity and 16% against it in a border poll.
Fewer than one in 10 Sinn Fein voters (9%) said the NHS would make them more likely to vote for the Union compared to 28% of SDLP voters.
The poll of 1,970 people was conducted by LucidTalk from September 25 to 28. The sample was weighted to reflect Northern Ireland's population.
A third of those aged over 65 - compared to just one in five 18-24 year-olds - said the health service would make them more likely to opt to remain in the UK.
But the poll shows under-45s choosing Irish unity by a significant margin regardless of the healthcare system. The results indicate that unionists must urgently face up to their ageing demographic. A total of 43% of 18-24 year-olds said that they would vote for Irish unity regardless of the NHS, compared to 34% who said the same about opting for the Union. The gap was even greater for 25-44 year olds - 42% said they would vote for a united Ireland and 29% to stay in the UK regardless of the NHS.
But 45-64 year-olds were split evenly on 35% for each option. The over-65 age group was the only one where more people said they would vote for the Union (40%) than for Irish unity (18%) regardless of the NHS.
The poll found that 84% of Sinn Fein and 56% of SDLP voters would definitely vote for a united Ireland, and 75% of DUP and 55% of Ulster Unionist voters would definitely vote to remain in the UK, regardless of the health service.
The poll also showed a significant difference between middle-class and working-class voters, with the former much stronger in their support for Irish unity. A total of 42% of ABC1 voters said they would definitely vote to leave the UK compared with 20% who said they'd vote to stay. Of C2DEs, 36% said they'd definitely vote for Irish unity, while 41% said the same for the Union.
The survey was commissioned from LucidTalk by political analyst, Mev Brown.
"It indicates that the NHS will be a key part of the political debate during a future border poll," he said.
"Deeply committed nationalists will opt for Irish unity regardless, but a significant number of SDLP supporters are more likely to be swayed to remain in the UK because of the healthcare system it offers. The NHS is an even bigger issue for Alliance and Green voters. That's not surprising because it is a much treasured institution providing cradle to grave healthcare based on clinical need, not the ability to pay.
"Potentially losing it doesn't overly concern the young, but it features much more prominently in the minds of older voters."
Mr Brown, who campaigned for Scotland to remain in the UK in the 2014 referendum there, said: "The NHS was a big issue in the Scottish referendum six years ago.
"Those supporting the Union said independence threatened NHS funding while the SNP argued that the threat of privatisation from a Conservative government was the biggest danger to the health service."
Sinn Fein leader Mary Lou McDonald has said that healthcare is usually the first issue raised with her by people of all persuasions in relation to a border poll.
She was speaking after Taoiseach Micheal Martin said a referendum would not be on his government's agenda for the next five years.
Former DUP leader Peter Robinson has said that unionists must prepare for a border poll. He branded "border poll deniers", who think the vote will never take place, of indulging in "complacent and dangerous thinking".