The principle of requiring unionist consent to constitutional change affecting Northern Ireland has been breached for the first time over Brexit, Arlene Foster has said.
Loyalists are adamantly opposed to what they term the proposed creation of a customs border with the rest of the UK under Boris Johnson's draft Brexit deal with the EU.
Extra administration could be involved in sending goods from Great Britain to Northern Ireland in a bid to avoid a hard regulatory border on the island of Ireland.
The DUP leader said: "We have to trust ourselves and know what we want to get out of any deal and the issues around customs, goods coming from the mainland to Northern Ireland, and indeed goods going from Northern Ireland to the mainland, is a key issue that we need to get clarity on."
The party launched its general election manifesto in Belfast today.
The manifesto said east-west checks as proposed would lead to "excessively bureaucratic" burdens for trade between Great Britain and Northern Ireland and consequently higher prices and less choice for consumers as British businesses opt not to sell in the north.
Ms Foster said she is seeking clarity around the consent mechanism.
She said: "The current consent mechanism goes against the Belfast Agreement and we have been told for some time now that when there are fundamental issues, there has to be agreement from the majority of unionists and the majority of nationalists; this is the first time that has been breached and we cannot accept that."
Loyalists have held meetings opposed to what they term the creation of an "economic united Ireland" under the British prime minister's deal.
Reference to an "armed struggle" was made during one recently in Portadown, Co Armagh, if Northern Ireland's position in the UK is threatened.
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Ms Foster said her party's Upper Bann General Election candidate Carla Lockhart was present and immediately interjected that violence would not be the case.
The party leader added: "We abhor violence and the use of violence, always have done, and always will do.
"We do not have any truck with anybody who would advocate that sort of thing, therefore we stand on law and order and respect for the law and everybody is equal under the law and we have always been very clear about that."
Deputy party leader Nigel Dodds criticised Boris Johnson's handling of Brexit negotiations.
He said: "After this is done there should be a full proper inquiry set up at a high level into what went wrong with the British government negotiations. How can we have such a catastrophic conduct of negotiations in the most important issues of our time?"
Ms Foster also said she would not support a Jeremy Corbyn-led administration after next month's election and referred to the anti-Semitism row.
She added: "It is not just about the company he kept in the past, it is what he believes now.
"It is not just about the person, it is about his policies, from how he is going to wreck the economy and take us back to a time when we did not have productivity, where people were leaving the UK.
"I think that is a bad place to be, and of course we are concerned about the defence of our nation as well and we would be very worried about the fact that he would be in control of all of that."
