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.

Brexit threatens life on the Irish border: in pictures Show all 15 1 /15 Brexit threatens life on the Irish border: in pictures Brexit threatens life on the Irish border: in pictures An abandoned shop is seen in Mullan, Co Monaghan. The building was home to four families who left during the Troubles. The town was largely abandoned after the hard border was put in place during the conflict. Mullan has seen some regeneration in recent years, but faces an uncertain future with Brexit on the horizon Reuters Brexit threatens life on the Irish border: in pictures A defaced ‘Welcome to Northern Ireland’ sign stands on the border in Middletown, Co Armagh Reuters Brexit threatens life on the Irish border: in pictures Mervyn Johnson owns a garage in the border town of Pettigo, which straddles the counties of Donegal and Fermanagh. ‘I’ve been here since 1956, it was a bit of a problem for a few years. My premises has been blown up about six or seven times, we just kept building and starting again,’ Johnson said laughing. ‘We just got used to it [the hard border] really but now that it’s gone, we wouldn't like it back again’ Reuters Brexit threatens life on the Irish border: in pictures Farmer Gordon Crockett’s Coshquin farm straddles both Derry/Londonderry in the North and Donegal in the Republic. ‘At the minute there is no real problem, you can cross the border as free as you want. We could cross it six or eight times a day,’ said Crockett. ‘If there was any sort of obstruction it would slow down our work every day’ Reuters Brexit threatens life on the Irish border: in pictures John Murphy flies the European flag outside his home near the border village of Forkhill, Co Armagh Reuters Brexit threatens life on the Irish border: in pictures Potter Brenda McGinn stands outside her Mullan, Co Monaghan, studio – the former Jas Boylan shoe factory which was the main employer in the area until it shut down due to the Troubles. ‘When I came back, this would have been somewhere you would have driven through and have been quite sad. It was a decrepit looking village,’ said McGinn, whose Busy Bee Ceramics is one of a handful of enterprises restoring life to the community. ‘Now this is a revitalised, old hidden village’ Reuters Brexit threatens life on the Irish border: in pictures Union Flag colours painted on kerbstones and bus-stops along the border village of Newbuildings, Co Derry/Londonderry Reuters Brexit threatens life on the Irish border: in pictures Grass reflected in Lattone Lough, which is split by the border between Cavan and Fermanagh, seen from near Ballinacor, Northern Ireland Reuters Brexit threatens life on the Irish border: in pictures Donegalman David McClintock sits in the Border Cafe in the village of Muff, which straddles Donegal and Derry/Londonderry Reuters Brexit threatens life on the Irish border: in pictures An old Irish phone box stands alongside a bus stop in the border town of Glaslough, Co Monaghan Reuters Brexit threatens life on the Irish border: in pictures Billboards are viewed from inside a disused customs hut in Carrickcarnon, Co Down, on the border with Co Louth in the Republic Reuters Brexit threatens life on the Irish border: in pictures Seamus McQuaid takes packages that locals on the Irish side of the border have delivered to his business, McQuaid Auto-Parts, to save money on postal fees, near the Co Fermanagh village of Newtownbutler. ‘I live in the south but the business is in the North,’ said McQaid. "I wholesale into the Republic of Ireland so if there’s duty, I’ll have to set up a company 200 yards up the road to sell to my customers. I’ll have to bring the same product in through Dublin instead of Belfast’ Reuters Brexit threatens life on the Irish border: in pictures A disused Great Northern Railway line and station that was for customs and excise on the border town of Glenfarne, Co Leitrim Reuters Brexit threatens life on the Irish border: in pictures Alice Mullen, from Monaghan in the Republic of Ireland, does her shopping at a former customs post on the border in Middletown, Co Armagh. ‘I’d be very worried if it was a hard border, I remember when people were divided. I would be very afraid of the threat to the peace process, it was a dreadful time to live through. Even to go to mass on a Sunday, you’d have to go through checkpoints. It is terribly stressful,’ said Mullen. ‘All those barricades and boundaries were pulled down. I see it as a huge big exercise of trust and I do believe everyone breathed a sigh of relief’ Reuters Brexit threatens life on the Irish border: in pictures A bus stop and red post box stand in the border town of Jonesborough, Co Armagh Reuters

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.