Boris Johnson will “abandon” the DUP if he wins an election, to secure a Canada-style free trade agreement for Britain, the UK’s chief negotiator in the Belfast Agreement talks has predicted.

Jonathan Powell said he believes the British prime minister is building an “English national party” and is in a strong position to win a snap general election which will mean he no longer needs DUP support.

The Conservatives are in government in Westminster with a razor-thin majority through a confidence-and-supply deal with the Democratic Unionists’ 10 MPs.

Mr Powell, former chief of staff to ex-prime minister Tony Blair, told RTÉ’s Marian Finucane show that he believes Mr Johnson is pursuing a Canada-style free trade agreement with the European Union after leaving the bloc which will mean Britain being outside the Single Market and the Custom’s Union.

“That means Northern Ireland would have to go back to the original backstop [guarantee against a hard border],” he said.

“There would definitely have to be special measures for Northern Ireland in that case and there would definitely be a border down the Irish Sea.”

Mr Johnson has only two options – for the EU to back down on his demand for the backstop to be scrapped, or to call a general election to shore up his own mandate, said Mr Powell.

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It is unlikely the EU will back down and Mr Johnson has a “good chance” of winning a majority in the House of Commons if the UK goes to the polls, unless opposition parties can come together to form a pact, he added.

Jonathan Powell: ‘The DUP has a relatively weak leadership under Arlene Foster.’ Photograph: Jonathan Porter/PressEye

“If he were to win to a majority, Boris Johnson has no particular fixed points and, yes, if he needed to abandon the DUP to get to a solution, so he could get to a Canada-style free trade agreement for the rest of Great Britain, then that is exactly what he will do,” Mr Powell said.

“He is turning the Tory party into an English national party.

“They are not interested in Scotland, they are abandoning the Tory party in Scotland which has a very different position and they are focusing entirely on trying to win seats in England. If they get that, those are the interests they will protect.”

Mr Powell, who helped to broker peace in the North, said Mr Johnson was pursuing a “clever strategy” and the DUP have a “problem politically”.

“The DUP has a relatively weak leadership under Arlene Foster,” he said.

“It is at least two parties. It is the party in Westminster and the party in Northern Ireland – they have two rather different interests.

“Someone like [DUP East Antrim MP and the party’s Brexit spokesman] Sammy Wilson likes hanging out in the bars with the ERG [European Research Group], the right wing of the Tory party, and they are very much signed up for Brexit,” he said.

“Those who have to contest local Assembly elections in Northern Ireland.. their support is more dubious about the virtues of a no-deal or hard Brexit.

“I think the DUP have a problem politically and they are very unlikely to back down on anything that undermines the union, but if it gets past that, they are in a more complicated position.”

Mr Powell also berated hardline Brexiteer MPs for throwing “childish” insults across the Irish Sea at Taoiseach Leo Varadkar and other Irish politicians,warning they were damaging hard-won advances in Anglo-Irish relations over recent years.

“I hope it is not a permanent set back,” he said, adding that previous bad relations had “very deep historical roots”, many of which, while not cured, had been at least put in some perspective.

“There will be some rebuilding to do once the Brexit issue is resolved,” he said.