DUP leader Arlene Foster has told a press conference her party "will not accept any regulatory divergence that will separate Northern Ireland economically or politically from the UK".
She also accused the Irish government of "seeking to unilaterally change the Belfast Agreement without our input or consent".
Delivering her statement in the Great Hall at Stormont, Mrs Foster was responding to reports that Theresa May's government were preparing a major compromise on the Irish border that would see "regulatory alignment" between Northern Ireland and the Republic.
No questions were taken after she addressed reporters and cameras gathered in Parliament Buildings. The DUP is likely to face severe pressure over the leaked proposals, with TUV leader Jim Allister describing the mooted proposals for regulatory alignment as "a stepping stone to a united Ireland".