SDLP leader Colum Eastwood has said there would be a special place in hell for those who call for a referendum on Irish unity without a plan.
Mr Eastwood said a border poll should not be held until work to build a new and reconciled Ireland was completed.
Sinn Féin has repeatedly called for a border poll to be conducted should the UK crash out of the EU without a deal.
He made the remarks during an address at the Fianna Fáil Ard Fheis in Dublin on Saturday.
Read More: DUP letter urges unionists to prepare for border poll
It was his first address at the conference since the two parties announced a partnership in January.
"Nationalism is not ready for a border poll."
— The Nolan Show, BBC (@BBCNolan) February 25, 2019
SDLP leader @columeastwood hits out at Sinn Fein's call for a border poll after #Brexit and says there is "a special place reserved in hell" for those calling for a border poll with no plan.
https://t.co/xoPAknadt1 pic.twitter.com/ssB8HUPdic
Mr Eastwood told delegates at the conference in Citywest that a new Ireland could only be reached if definition and detail were provided.
"There will be a special place reserved in hell for those who call for a border poll in Ireland with no plan and idea," he said.
The Foyle MLA added that a key role for the new political partnership was to begin a conversation with unionists over what a new Ireland would look like.
"We recognise the deep divisions that exist across our community and we know that it will take real leadership to begin to bring our communities together," he said.
He also said that there was no hope of reconciliation without an assembly and that nationalists on the island of Ireland "must not be seduced or misled" into the narrative that powersharing was a convenience rather than a necessity.
"There is no hope of delivering integrated economic and social progress across this island without an assembly at Stormont," he said.
"There is no way of fully protecting our people against the devastating consequences of Brexit without an assembly in Stormont."
Mr Eastwood said a hard border between Northern Ireland and Ireland threatened to bring "existential threats" to many industries in Northern Ireland and also threatened to position the region as a permanent economic backwater.
"Any hardening of the border will be a deliberate violation of our political process by the British Government," he said.
"They have simply no right.
"In Northern Ireland, the Good Friday Agreement is sovereign.
"Both of our parties know this is true - we were the architects of that agreement, we built it.
"That agreement belongs to the people of Ireland and we are the only people with the right to change it."
Speaking about, Brexit, Mr Eastwood described Brexit it as the "ever-shifting water beneath our feet".
He told delegates that no-one should be in any doubt that the instability of Brexit and the instability faced by the institutions of the Good Friday Agreement were inextricably linked.
"A hard border in Ireland threatens to bring existential threats to many of our industries in the north.
"It threatens to position us as a permanent economic backwater.
"This is particularly true for communities west of the Bann and along the border... All of us instinctively know that this can't be allowed to happen."
Discussing Fianna Fail's partnership with the SDLP, leader Micheál Martin said a new political agenda was required in Northern Ireland.
"Sinn Fein and the DUP, in our view, have essentially - by the way they ran the executive - undermined the spirit of the Good Friday Agreement," he said.
He said he believed a new middle ground was emerging in the region that was not obsessed with constitutional issues and instead wanted to see the institutions of the Good Friday Agreement functioning.
"The sensible thing for us is to work with the SDLP who are routed in the north ... but work with them on a policy agenda in terms of that bringing new ideas and approaches," he said.
