He issued the warning during a debate on the Northern Ireland (Ministers, Elections and Petitions of Concern) Bill at Westminster this afternoon.
"What is really concerning, all these years after the Good Friday agreement, is that as of today, none of the Unionist parties has told us what they would do if a nationalist gets enough votes to occupy the First Minister’s position.
"They are refusing to tell us whether they would even serve in that Government. Well, it is not 1968 anymore, and nationalists will no longer be treated as second-class citizens. People have marched in the streets and been beaten off the streets so that our votes could count just as much as anyone else’s.
"If Unionist politicians want to come along and lecture anybody about the sustainability of institutions and working together, they must seriously consider their answer the next time they are asked whether they would serve as Deputy First Minister if a nationalist becomes First Minister," he said.
The SDLP leader predicted the run-up to the election will be dominated by speculation around whether the next First Minister will be a nationalist or a unionist.
"From listening to some of the big radio shows in Northern Ireland and watching the television news, it is clear that over the next six months in the run-up to this election—if we are allowed to have an election—we will be faced with constant arguing: 'Who will be First Minister and who will be Deputy First Minister? You have to come out to vote to stop these people becoming First Minister.'
"Even though we have had that for 20 years, the DUP still go into government with them. DUP Members used to say, 'We can’t have Martin McGuinness as First Minister. He was a terrorist', but then they went into government with him, occupied that very same office, and worked with him every day.
"Let us, please, get rid of the constant division and debate about who is First Minister and who is Deputy First Minister," he said.
Mr. Eastwood claimed 'it suits the DUP and Sinn Féin to have constant debate about what they call each other, because then we are not dealing with the real issues'.
"Our health service is on the point of collapse, 100 times more people are on out-patient waiting lists in Northern Ireland than they are in England, 29% of our children are living in poverty, but there is still no antipoverty strategy because they could not agree it.
"My constituency has the highest level of unemployment and economic inactivity anywhere across these islands, and we still do not have the 10,000 students on the Magee university campus who were promised and negotiated by me and the former Secretary of State for Northern Ireland during those New Decade, New Approach (NDNA) discussions."
The Derry MP said it was ironic to see the British Government fulminating over political instability in Ireland given how they are handling Brexit.
"In conclusion, it is a bit rich for the Government to be telling anybody about sustainability in Northern Ireland, when everything they do in Northern Ireland undermines sustainability and the stability of our institutions.
"That includes how they dealt with the European Union and the DUP, and what they told them about the protocol—apparently there was never going to be a border anywhere. Well, there is one now, and if we were more honest with people we would be in a much better situation."