The potentially dire consequences of a hard Brexit have been spelled out in a letter from the SDLP to Labour leader Jeremy Corbyn.
Urging Mr Corbyn to back full access and full alignment with the single market and customs union, SDLP leader Colum Eastwood has also outlined the possibility of destabilising the Northern Ireland peace process.
He said: “My message to Mr Corbyn is simple, protect that positive Labour legacy rather than allowing a hard Brexit to destroy it.”
The majority of people in Northern Ireland voted to stay within the EU during the 2016 referendum.
Some 56% backed remain while 44% voted to leave.
Following a visit to Londonderry and Donegal last month, shadow Brexit secretary Sir Keir Starmer warned the government must keep to solemn commitments, made during the first phase of Brexit negotiations, to avoid a hard border.
Mr Eastwood wrote: “As you know, your party invested huge amounts of effort and time in helping to secure the peace process through the institutions of the Good Friday Agreement.
“Brexit threatens all that was achieved in that historic accommodation; it threatens the great positivity of that Labour legacy. Those institutions are currently experiencing severe strain, the imposition of a hard Brexit carries with it the potential to dismantle the architecture of our peace process. “
Mr Eastwood also called for Mr Corbyn to show leadership on the matter.
“Many of us in Ireland have been greatly encouraged by the evolving position your party has adopted since the Brexit referendum result. Having come this far, I would urge you to take the next progressive step and advocate continued access to the European single market and customs union.
“In doing so, you will go a long way in protecting the historic progress that has been made over so many years in Northern Ireland. It would prove a decisive moment of leadership which holds the potential to finally turn the Brexit tide in all of our favours. It would be a moment of leadership which I firmly believe the peoples of Ireland and Britain will come to thank you for.”