Arlene Foster has said the Good Friday Agreement is not "sacrosanct", insisting the landmark peace treaty could be altered to accommodate a Brexit deal.
The DUP leader, whose party props up Theresa May's government, said the historic agreement "could evolve" in the EU context and expressed frustration at warnings from pro-EU campaigners over Brexit's potential to undermine aspects of the international treaty.
Ahead of a speech on the fringe of the annual Tory conference, Ms Foster praised Boris Johnson's "positive" Brexit vision and said she would work with him if he became prime minister.
She also poured cold water on Ms May's idea of having regulatory checks in the Irish Sea, saying: "It's our one red line".
Critics accused Ms Foster of "reckless and desperate stuff" and condemned her for prioritising her party's "cosy deal" with the Conservatives over the ongoing risk of violence in Northern Ireland.
She told The Telegraph: “It has been deeply frustrating to hear people who voted Remain and in Europe talk about Northern Ireland as though we can’t touch the Belfast Agreement.
"Things evolve, even in the EU context. There has been a lot of misinterpretation, holding it up as a sacrosanct piece of legislation.”
The DUP campaigned against the Good Friday Agreement in 1998, when it was approved by referenda on both sides of the border.
The former first minister also endorsed the "spirit" and "belief" in Mr Johnson's Brexit plans, ahead of the ex-foreign secretary's eagerly-anticipated speech to conference, which is being interpreted by many as a leadership bid.
Ms Foster said: “I think people want that hope, they want to be positive. I think the reason why so many people are turned off by Brexit is because they are being fed a diet of negativity - whether it’s infighting, Brussels, being disrespected by people over there.
“We haven’t been able to talk about the aspirations for the nation, we’ve spent so much time arguing about what’s happened, is it going to be a disaster for Ireland in inverted commas...instead of actually focusing on what we can achieve in the UK with the Brexit negotiations.
“What we want to see, and I’m not making a comparison between Boris and the Prime Minister, is belief. We want to see that spirit.”
Asked if she would work with Mr Johnson, Ms Foster said: “Our confidence arrangement is with the Conservative Party. It was signed by the two chief whips. It is a party-to-party agreement.
“Whoever leads the Conservative Party we will work with as it’s in the national interest. The reason we signed the agreement was to ensure Brexit.”
It will come as a boost to Mr Johnson, after the party's Brexit spokesman Sammy Wilson initially described his suggestion of a Canada-style trade deal as "too vague".
In a separate interview with Bloomberg TV, Ms Foster torpedoed the prime minister's plan for regulatory checks in an attempt to resolve the Irish border wrangle.
Asked if her party could accept such checks, she said: “No and it’s been very clear all along that has been our one red line - that we cannot have either a customs border down the Irish Sea or a regulatory border because that would make us separate from the rest of the United Kingdom and that doesn’t work from a constitutional perspective and it doesn’t work from an economic perspective either.”
Political opponents roundly condemned the DUP leader's comments.
Former shadow Northern Ireland secretary, Owen Smith, said: "It now seems that delivering a hard Brexit and protecting their cosy deal with the Tories is sacrosanct for the DUP, but sustaining the Good Friday Agreement is not.
"The open border in Ireland is not tangential to the Good Friday Agreement, it is central to its spirit and ethos.
"On the very day when PSNI officers are warning about the ongoing risks from paramilitaries, this is reckless and desperate stuff from Arlene Foster and the DUP."
Sinn Fein president Mary Lou McDonald insisted the 1998 accord must not become a bargaining chip in the Brexit negotiations.
She said: "Today's comments by DUP leader Arlene Foster on the Good Friday Agreement are unacceptable and reveal a reckless disregard for the peace process, prosperity and progress."
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