Updated: 6:24 p.m. EDT

Two decades of peace in Northern Ireland could be at risk if the United Kingdom’s new government relies on the support of Ulster’s Democratic Unionist Party, which represents one of the two formerly warring sides, former Prime Minister John Major said on Tuesday.

“I think the peace process is fragile,” Major, a former Conservative Party leader, told the BBC. Major shared his concern as his party’s current leader, Prime Minister Theresa May, was working to strike a deal with the DUP leader, Arlene Foster, that would allow the Conservatives to form a government with the support of the Ulster unionists.

During his time in office, Major helped to start the process that eventually led to the end of violence and a finely balanced local assembly in Belfast — where power is shared between the mainly Protestant unionist community, which celebrates its British heritage, and the mainly Catholic nationalist community, which identifies as Irish.

“A fundamental part of that peace process is that the U.K. government needs to be impartial between all the competing interests in Northern Ireland,” Major said. “And the danger is that however much any government tries, they will not be seen to be impartial if they are locked into a parliamentary deal at Westminster with one of the Northern Ireland parties.”

“The last thing anybody wishes to see,” Major added, “is one or other of the communities so aggrieved that the hard men, who are still there lurking in the corners of the community, decide that they wish to return to some form of violence.”

That concern is made more pressing by two factors: the looming threat of Brexit, which could make it necessary to once again erect barriers along the currently invisible border between Northern Ireland and Ireland, once the U.K. leaves the European Union; and the fact that Northern Ireland’s local assembly is in crisis.

Unlike the other major parties in Northern Ireland, and 56 percent of the region’s voters, the DUP supported Brexit — apparently seeing the gradual erosion of borders between the part of the island still in the U.K. and the Republic of Ireland as an existential threat. At the same time, the party’s voters have clearly benefited economically from the effective dissolution of the border, and from E.U. development funds. That might explain the otherwise puzzling promises in the DUP’s election manifesto to work for a “Frictionless border with Irish Republic,” and to retain the “ability to opt-in to EU funds.” As a party, then, the DUP view of Brexit seems to be the same as that of the foreign secretary, Boris Johnson, who joked that his “policy on cake is pro having it and pro eating it.”

The local government, meanwhile, has not been in session for months, since Sinn Fein, the former political wing of the Irish Republican Army, withdrew its cooperation with the DUP, which is supported by former loyalist paramilitaries, over Arlene Foster’s mismanagement of a renewable energy program that could end up costing Northern Ireland hundreds of millions of pounds.

If, however, the new U.K. government can only function with unionist support, it is indeed hard to see how it could also act, in concert with the Irish government in Dublin, as a neutral arbiter in talks between the two sides in Ulster.

“The idea that Theresa May — or any Tory politician — can serve as an impartial mediator while relying on D.U.P. votes at Westminster is a joke in very poor taste,” Daniel Finn wrote on the London Review of Books website on Monday. “A parliamentary alliance between the Tories and the D.U.P. will reinforce an ideological convergence between the parties,” he added. After all, as Finn pointed out, May’s Conservative and Unionist Party, to give it its full name, also tried, in its recent election manifesto, to pin the blame for the killing of more than 3,000 people during “the Troubles” in Northern Ireland, entirely on the I.R.A. “The immense contribution of the security forces during the Troubles should never be forgotten,” the Conservative manifesto read. “We will reject any attempts to rewrite history which seek to justify or legitimise terrorism,” the document promised — conveniently forgetting about the murder of many Irish Catholic civilians by the security forces and their allies in loyalist paramilitary groups close to the D.U.P.

As Gerry Adams, the Sinn Fein leader, noted on Monday, the outgoing Irish prime minister, or taoiseach, Enda Kenny, called the British prime minister over the weekend to express his concern that her new alliance could disrupt their efforts to get the legislature in Belfast back on track.

There have even been suggestions that such an alliance in the British Parliament could be a violation of the terms of the 1998 Belfast Agreement, which stipulates that the U.K. must remain impartial in Northern Ireland.

Apart from legal wrangling, though, there is also concern that the unionists might seek to extract concessions from the Conservatives that would immediately exacerbate sectarian tensions in Northern Ireland. Some DUP voters even seem anxious to make that a reality.

The Orange Lodge of Portadown, a fraternal order of anti-Catholics that counts among its members David Simpson, one of the DUP’s 10 members of Parliament, issued a statement over the weekend calling on the party to use its leverage with the prime minister to get her to permit its annual parade to once again pass through Catholic areas.

The Orange order’s July 12 parade, celebrating the victory of the Protestant King William of Orange over the Catholic James II at the Battle of the Boyne in 1690, has been barred from passing through a Catholic area by the authorities since 1998, when three Catholic children were murdered by rioting unionists.

Apart from the potential threat to peace in Northern Ireland, some members of the opposition Labour Party have spoken out against giving the DUP a formal role in the new U.K. government because of its regressive social views, including its opposition to same-sex marriage.