The Democratic Unionist Party (DUP) has said it “could not support” Prime Minister Boris Johnson’s revised Brexit plan for Northern Ireland.
Party leader Arlene Foster and deputy leader Nigel Dodds said on Thursday they were in talks with the government.
“As things stand we could not support what is being suggested on customs and consent issues and there is a lack of clarity on VAT,” they added.
They said they would work with the government to “get a sensible deal”.
The BBC’s Europe editor Katya Adler said the EU’s response was that the DUP’s refusal to back the plan was a problem for the UK.
A Brexit deal under consideration in Brussels would involve Stormont giving ongoing consent to any special arrangements for Northern Ireland.
The Stormont role would not be the unionists’ veto demanded by the DUP – instead the arrangements could be approved by a straight majority.
Pro-EU parties have a narrow majority at Stormont.
The proposed Brexit deal would involve Northern Ireland being treated differently to the rest of the UK.