The DUP's new Westminster leader has urged British Prime Minister Boris Johnson to address the Northern Ireland customs arrangements in his Brexit deal.
Speaking in the House of Commons, Jeffrey Donaldson said: "We supported Brexit, and we want Brexit to happen and we acknowledge and recognise that the government has won a mandate to take forward its Withdrawal Agreement.
"But there is a major contradiction at the heart of that agreement which causes us great concern.
"At the one level the agreement does say that Northern Ireland should continue to have unfettered access to the rest of the UK for trade, but then we have customs arrangements that inhibit our ability to have that unfettered access, and that is our major concern, and one that we hope the government can address."
He added: "I want the Prime Minister to treat my part of the United Kingdom the same as the other parts in the context of leaving the European Union."
Mr Donaldson said that while he welcomes the fact that the Northern Ireland Assembly will have a say over special arrangements, he believes the arrangements "will continue unless the Assembly stops them" which requires a vote in the Assembly - and that there are "issues around how that would be exercised".
He added: "We do want the Assembly to have a say, but we want to ensure that say can be exercised in a fair manner that respects the principle of consent as set out in the Belfast Agreement."
He said: "We want Northern Ireland to benefit fully from any free trade agreement that is arranged with the European Union."
He continued: "But what we don't want, Mr Speaker, is barriers to trade with the rest of our own country - that is absolutely essential for us."
On customs arrangements, Mr Donaldson said "it is clear that those checks will take place".
He added: "We want to work with the Government to mitigate the impact on Northern Ireland business of the requirement for those checks, and again that's something we want to hear more from the government on and we will be looking towards committee stage to see what we can do to reflect the commitment in the agreement and the commitment from the Prime Minister that there will be unfettered access between Northern Ireland and Great Britain in relation to trade."
He said the DUP would "like to be in a position to support" the Prime Minister's deal, but "such is the gravity of our concern", they cannot at its present state.
Meanwhile, the DUP rejected claims that it is to blame for a failure to reach agreement to restore devolution at Stormont.
It was announced that while a potential agreement to restore devolution was close, it had not been possible to close the deal.
In a highly unusual move, the Irish and British governments pointed towards the DUP for the blame.
This morning, the DUP's eight MPs met Northern Ireland Secretary Julian Smith in London.
Afterwards the party firmly rejected the claim that it was to blame, and said it could not sign up to an agreement it has not yet seen.
If there is no agreement before 13 January, there could be a fresh Assembly election.
Meanwhile, SDLP MP Claire Hanna used her maiden speech in the House of Commons to raise issues about the impact of the UK's withdrawal from the EU, telling MPs the 2016 referendum created a "problem that didn't need to exist" and which "reopens old wounds".
Ms Hanna, outlining the SDLP's opposition to the European Union (Withdrawal Agreement) Bill, said: "For Northern Ireland in particular, Brexit has sharpened all of the lines that the Good Friday Agreement was designed to soften around identity, around borders, around sovereignty.
"We should have been spending the last few years talking about reconciliation, talking about regeneration, talking about social justice and equality.
"That's what all political action should really be about but instead we've spent morning, noon and night talking about Brexit - a problem that didn't need to exist and which, particularly in Northern Ireland, reopens old wounds and limits our horizons."
Ms Hanna said Brexit "fed off" people who felt lost and disenfranchised in the political system, adding: "I fear it will leave them feeling much worse."
She also said: "Beyond the economy, Brexit upends the delicate balance that in Northern Ireland has allowed us to imagine our shared and equal future together.
"We in Northern Ireland know the value of the EU and, as my political hero and predecessor in this House John Hume so often said, the EU is the greatest peace-building and conflict resolution project anywhere in the world and those of us particularly affected by conflict have a duty to reflect its principles.
"I'm afraid the concerns we have have been dismissed by those of you who won't ever have to live with the consequences of these actions."