Weariness over Brexit should not be an excuse for "weakness" on Brexit or the union, the DUP warned Boris Johnson.
Nigel Dodds, the DUP's Westminster leader, urged the prime minister to reconsider his Brexit offer and ensure the UK leaves the EU as one.
He voiced concerns over the impact of the proposed agreement on Northern Ireland and reminded Mr Johnson about previous commitments he had given.
Speaking in the House of Commons, Mr Dodds told the PM: "Weariness in this House over Brexit should not be an excuse for weakness on Brexit or weakness on the union."
Mr Dodds said there must be "Brexit for the whole of the United Kingdom", leaving the single market and customs union as one.
He said: "This deal puts Northern Ireland, yes, in the UK customs union but applies de facto all the European customs union code, yes it does, read the detail."
Mr Dodds raised concerns over Northern Ireland's involvement in the VAT regime and the single market "without any consent up front" before claiming: "It drives a coach and horses through the Belfast Agreement by altering the cross-community consent mechanism."
Mr Dodds then alluded to previous warnings from Mr Johnson about how no British PM could agree to such terms, adding: "Will he now abide by that and please reconsider the fact that we must leave as one nation together?"
Mr Johnson, replying to Mr Dodds, said together he and the DUP secured changes on the customs union before defending the measures in the deal for Northern Ireland.
He said: "In all frankness I do think it a pity that it is thought necessary for one side or the other in the debate in Northern Ireland to have a veto on those arrangements.
"Because after all, I must be very frank about this, the people of this country have taken a great decision embracing the entire four nations of this country by a simple majority vote that went 52-48, which we're honouring now.
"I think that principle should be applied elsewhere and I see no reason why it should not be applied in Northern Ireland, and it is in full compatibility with the Good Friday Agreement."
Independent MP Lady Hermon said: "The Prime Minister will be well aware of the considerable anxiety and indeed anger caused to the unionist community in Northern Ireland since the publication of his Brexit deal.
"I would like the Prime Minister to take this opportunity, since he has not bothered until now, to reach out and reassure the unionist community, I would like him to take this opportunity, publicly, to reassure the people in Northern Ireland that there is nothing in his deal which undermines the constitutional status of Northern Ireland as guaranteed by the Belfast/Good Friday Agreement and the consent principle.
"I pay tribute to the Taoiseach Leo Varadkar who has made a very, very solemn explanation and commitment about his commitment to the constitutional status of Northern Ireland, but as a unionist, I need to hear a British prime minister making that commitment to the unionist community."
Mr Johnson replied: "I am grateful to the right honourable lady and I wish to reassure her that I make an absolute commitment to the constitutional status of Northern Ireland which is inviolable and in tact.
"And it may be that she hasn't seen the statement from Lord Trimble who said of the change in the agreement that we have secured: 'Whilst previously the people in Northern Ireland were to have an agreement imposed on them, now we have a mechanism for the consent of the people of Northern Ireland and that is fully in accordance with the Good Friday Agreement'."
DUP to consider backing Letwin amendment
Earlier, the DUP said it would consider backing an amendment which would delay any immediate decision on Mr Johnson's Brexit deal when it came before parliament today.
"Were going to look at that very closely," Nigel Dodds, deputy leader of the DUP, told BBC radio when asked if they would support the amendment put forward by expelled Conservative politician, Oliver Letwin.
"We will certainly look at it very, very carefully."
"We will not be supporting the Government, we will be voting against.
"Because it isn't Brexit for the whole of the United Kingdom", he said.
His party had already said it would not vote in favour of the deal because of its customs implications for Northern Ireland.
"It does have the merit of pointing out that this would withhold approval of the Commons from the government's plans.
"So, we haven't made any final decision on that, we will meet later and discuss as a parliamentary group... tactics."
Former Tory minister Oliver Letwin is now sitting as an Independent.
The Scottish National Party tabled an amendment to reject the deal demanding an immediate extension to the 31 October deadline and a general election.