The European Union wants Northern Ireland to have a different Brexit deal from the rest of the UK, papers seen by the BBC suggest.
The document says the UK should take responsibility for finding a “unique solution” so people can work, go to school or get medical treatment either side of the Irish border.
Details will be published by the EU’s chief negotiator Michel Barnier later.
He has said “a lot more substantial work” is needed on the border issue.
The impact of Brexit on Northern Ireland – and its border with the Republic of Ireland – is one of the key issues being discussed in the early stages of UK-EU negotiations.
Fears have been raised that a return to border checks could undermine the Good Friday peace agreement and damage the economy.
The UK has said it wants an “unprecedented solution”, avoiding physical checks at the border.
But no agreement has been reached so far on how this can be achieved, given the UK’s decision to leave the EU’s customs union after Brexit.
The EU’s proposals will be set out later as part of a series of “position papers”.
The paper on Northern Ireland will say: “The onus to propose solutions which overcome the challenges created on the island of Ireland by the United Kingdom’s withdrawal from the European Union and its decision to leave the customs union and the internal market remains on the United Kingdom.”
It adds: “These challenges will require a unique solution which cannot serve to preconfigure solutions in the context of the wider discussions on the future relationship between the European Union and the United Kingdom.”
Brussels has refused to discuss the UK’s future relationship with the EU – notably how they will trade with each other – until the initial discussion issues, including Northern Ireland, have been settled.
The EU’s paper also says nothing can affect Ireland’s place in the EU’s single market and customs union.
And it suggests specific provisions being written into the final departure deal to protect cross-border co-operation in areas like health, education, transport and fishing.
The Liberal Democrats said the EU’s document “demolishes another of the Leave campaign’s fantastical claims – that Brexit would have no impact on the Irish border”.
MP Tom Brake said the only solution to the border question was for the UK to stay in the single market and customs union.
The UK government released its own position paper on Northern Ireland last month.
The release of the EU’s papers comes as MPs prepare to debate the EU Withdrawal Bill in Parliament. The legislation will convert all existing EU laws into domestic ones ahead of Brexit – due to take effect at the end of March 2019.
Ministers said passing the legislation would form an “essential foundation” for post-Brexit Britain, but Labour has vowed to vote against it as it stands, calling it a power grab by the government.