The EU's chief Brexit negotiator has repeated that a UK decision to leave the single market and customs union would make Irish border checks "unavoidable".
Michel Barnier was speaking in Brussels.
He made his comments at the conclusion of a week of technical discussions between civil servants on both sides.
Earlier this week, Downing Street said the UK would not be in a customs union after Brexit.
Mr Barnier also said the UK's agreement that it could continue to follow some EU rules to prevent a hard border needed to be included in the withdrawal agreement.
He also warned that without a withdrawal agreement there could be no transition period.
In December, the UK said it would prevent a hard border either in the context of an overall deal, a special deal for Northern Ireland, or by continuing alignment with some EU rules.
Mr Barnier said the EU must now start legally defining how the alignment scenario would work and "there must be no ambiguity here".
He added that based on the discussion this week, the UK has accepted the necessity of discussing how to make this alignment scenario operational, provided the other two options are discussed in parallel.
The SDLP's Brexit spokesperson, Claire Hanna, said she welcomed Mr Barnier's "strong defence of Northern Ireland's economic, social and political interests".
"If Theresa May does not alter course, Michel Barnier is right to say in no uncertain terms that withdrawing from the single market and customs union will 'make border checks unavoidable'," she added.
Ulster Unionist MEP Jim Nicholson said that flexibility was needed in the Brexit negotiations and that "there need not be anything inevitable about border checks if a deep and comprehensive future relationship can be agreed in phase two of Brexit talks".
DUP MEP Diane Dodds said the "best vehicle for finding border solutions is via the future UK-EU relationship, through a comprehensive free trade agreement and a fresh customs partnership with Brussels".
"Everyone has committed to avoiding a hard border and the UK has said it will not impose physical infrastructure at the border," she said.
"It seems it is only the EU that is brandishing the threat of customs controls."
Barnier says decision to leave CU & SM would make border checks inevitable— JPCampbellBiz (@JP_Biz) February 9, 2018
On what was agreed in December: the alignment part of Section 49 has to be in the text of the withdrawal agreement. Have to define how it would work in operational terms. ‘No ambiguity.’— JPCampbellBiz (@JP_Biz) February 9, 2018