Arlene Foster’s party said the deal struck in Brussels was not in Northern Ireland’s long-term interest and drove “a coach and horses” through the Good Friday Agreement which underpins the peace process.
The loss of the votes of the 10 DUP MPs will make it all but impossible for Mr Johnson to get his deal through Westminster, particularly as several members of the European Research Group of eurosceptic Tories are likely to take their lead from Ms Foster.
In a statement issued shortly after Mr Johnson and European Commission president Jean-Claude Juncker announced they had reached agreement, the DUP said bluntly: “These proposals are not, in our view, beneficial to the economic well-being of Northern Ireland and they undermine the integrity of the Union.”
And they said: “The Democratic Unionist Party will be unable to support these proposals in parliament.”
Ms Foster has been the subject of a concerted sweet-talking campaign from Mr Johnson over the past few days, as the PM attempted to persuade her that his new plan did not breach the Unionist party’s red line of ensuring that Northern Ireland is not treated differently from the rest of the UK.
But she made clear her objection in an early-morning tweet stating that the DUP could not support his proposals on customs and consent “as things stand”.
The terse statement was followed by a lengthy evisceration of multiple elements of the deal unveiled by Mr Johnson and European Commission president Jean-Claude Juncker.