Forty-nine MLAs - including four from the Foyle constituency - have written to the European Council President Donald Tusk expressing their support for the 'backstop' and their grave concern over the prospect of a 'no deal' Brexit.
SDLP leader Colum Eastwood, his party and constituency colleague, Mark H. Durkan and Sinn Féin's local MLAs Karen Mullan and Raymond McCartney were among the signatories of the strongly worded letter that warned of the impact a crash-out would have on "our economy, our border and community cohesion".
Former Mayor of Derry City and Strabane District Council, Maolíosa McHugh, who is now a Sinn Féin MLA for West Tyrone also signed the letter.
The letter was backed by all Sinn Féin, SDLP, Alliance Party and Green Party MLAs, who represent 54 per cent of the electorate in the North.
It was not signed by any of the traditional unionist parties or by the Independent Unionist Claire Sugden.
Nor was it signed by the People Before Profit's sole MLA Gerry Carroll.
"An analysis carried out by the Northern Ireland Civil Service has predicted job losses in the region of 40,000 if we’re forced from the EU without a deal.
"The British government’s own analysis has predicted significant disruption to integrated supply chains across this island rendering cross-border trade in our agri-food sector virtually impossible for many operators. It has also warned that the open border that has underpinned our political settlement would be unsustainable within months," the MLAs write.
The pro-Remain MLAs warn that much of the progress made in developing relationships on this island could be lost if missteps were taken.
"With that in mind, and with no functioning Executive or Assembly currently in place in Northern Ireland to give expression to the democratic wishes of people here, we write to you to confirm our support for the backstop contained within the Withdrawal Agreement and Political Declaration.
"It is our view that a legally operable guarantee to protect the Good Friday Agreement, maintain north-south co-operation and preserve the all-island economy and to prevent a return to physical infrastructure on our border or physical checks at or near the border is necessary to preserve the progress that we have made," they state.