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 Fein 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 Fail national conference in Dublin on Saturday.
It was his first address at the conference since the two parties announced a partnership in January.
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 added that both the SDLP and Fianna Fail would fight to ensure that government returned to Northern Ireland.
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.”