The UK Government has put no serious thought into resolving the challenges of a hard Brexit for Northern Ireland, the SDLP said.
Brexit Secretary David Davis has written to the Stormont parties to give details of his priorities for the negotiations with Europe.
On the land border with the Irish Republic his letter said the Government wanted to avoid creating any new burdens that put Northern Ireland's businesses at a competitive disadvantage.
SDLP leader Colum Eastwood said: "The tunnel vision of Theresa May's Cabinet, pressing for a hard Brexit against the wishes of people in Northern Ireland and at a time when we have no functioning executive, is a matter of grave concern.
"The repetition of meaningless mantras about 'borders of the past' won't assuage the very real sense that this Government hasn't recognised the scale of the threat posed to Northern Ireland's stability by Brexit and, worse, that they don't really care.
"The issues outlined by David Davis in his letter are the issues we have been flagging with the British Government for months now.
"And while recognition is a first step, there seems to have been no serious thought put into how those challenges are resolved."
Businesses and local communities have voiced concern that Brexit could mean bureaucracy and checks at the border.
Mr Davis said: "We recognise the vital importance to Northern Ireland's economy of maintaining the open border for people and goods, both across the island of Ireland and within the UK.
"We wish to avoid creating any new burdens that put Northern Ireland's businesses at a competitive disadvantage.
"We know that, for the people of Northern Ireland, the ability to move freely across the border is an essential part of daily life."
He said the open border had an important and unique economic, social, political and psychological significance.
"We are determined that our future relationship with the EU recognises the importance of this and avoids the border becoming an impediment to movement and trade.
"We will prioritise, within our negotiations with the EU and in preparations for post-exit, an outcome that means the land border between the UK and Ireland is as seamless and frictionless as possible.
"We want to have the greatest possible tariff-free and barrier-free trade with our European neighbours, as well as to negotiate our own trade agreements around the world."
He recognised in particular the challenges faced by those industries with complex cross-border supply chains, physically separate from the rest of the UK, like agrifood, with its distinct animal and plant health regimes on the island of Ireland.
"We will continue to work closely with the new executive to establish a clear understanding of the key issues and to minimise the impact of non-tariff barriers on Northern Ireland business."