The DUP has accused Boris Johnson of losing his nerve and striking a bad Brexit deal in a desperate bid to avoid an extension.
Deputy leader Nigel Dodds said Mr Johnson had been “too eager by far” to come to an accommodation “at any cost” with the EU.
His comments came as DUP leader Arlene Foster accused London, Brussels and Dublin of turning their back on powersharing arrangements that form the cornerstone of the Good Friday/Belfast peace agreement.
Mr Dodds said: “The Benn Act has forced Boris Johnson into somewhat of desperation measures in order to avoid trying to get an extension.
“He has been too eager by far to get a deal at any cost, and the fact of the matter is, if he held his nerve and held out he would, of course, have got better concessions that kept the integrity, both economic and constitutionally, of the United Kingdom.”
Ms Foster said the agreement created several economic borders down the Irish Sea, which separated Northern Ireland from the rest of the UK.
“This gives us a border in the Irish Sea in terms of VAT, in terms of customs and in terms of single market rules, without any consent that is meaningful for the people of Northern Ireland,” she said.
The party leader said allowing Stormont to vote on retaining the arrangements on the basis of a straight majority vote, rather than using a peace process mechanism that requires a majority of unionists and nationalists, undermined the principle of powersharing.
“For the first time in 21 years we are moving away from powersharing, we are moving away from the majority of unionism and the majority of nationalists, we are moving to single majority vote,” she said.
“If we are going down this route in terms of majority rule, what does that mean for devolution?” she asked.
“What does it mean for the return of devolution, and all those things will have to be taken into account.”
She added: “All of that together makes this deal unacceptable for us as guardians of Northern Ireland in terms of the economy and in terms of the constitution.
“We wish it was otherwise but that, unfortunately, is the assessment we have made.”
Mr Dodds also criticised the consent arrangements.
“At the last minute the Irish Government, Europe and London are agreeing to drive a coach and horses through the very Belfast Agreement they profess to support, in order to ensure on this one single issue, of Northern Ireland being tied to the European customs union and single market rules, whilst the rest of the UK goes elsewhere in terms of its own regulations, that that has to be not by parallel consent, not by the consent of unionists and nationalists, but that one community can override the other.
“That clearly is contrary to everything they profess to believe.
“It is a major rewriting of the Belfast Agreement and it is something that anyone who has any concern for any kind of political process in Northern Ireland should be very, very concerned about.”