John Major knows exactly how delicate the balance of Northern Irish politics is. Theresa May should listen to him

The deal between Conservative Party and the DUP is a blatant breach of the promises made in the 1998 Belfast Agreement

Sir John Major understands the constitutional duty of a former prime minister, which is to draw on his experience and understanding to advise and warn his successors. He disconcerted David Cameron when, in 2013, he said he was appalled that “every single sphere of British influence” in society was still dominated by men and women who went to private school or who are from the “affluent middle class”. He did it again when he expressed his reservations about Mr Cameron’s promise of a referendum on EU membership.

Now Sir John has used one of his rare interventions to say he is “wary” and “dubious” about Theresa May’s imminent deal with the Democratic Unionist Party (DUP). This underscores what The Independent has been saying since Friday. We are concerned about the signals Ms May sends about women’s rights and LGBT rights by coming to an accommodation with the DUP, but we are also worried about the significant consequences for peace in Northern Ireland.

The UK Government undertook in the Belfast Agreement in 1998 to be impartial in Northern Irish politics. The deal between Conservative Party and the DUP, expected to be finalised on Wednesday or Thursday this week, is a blatant breach of that promise.

Sir John warned of “hard men still there, lurking in the corners of communities deciding they wish to return to some sort of violence”, and said that, although the peace settlement was unlikely to collapse quickly, it could “unwind” over time. “We need to do everything we conceivably can to ensure that doesn’t happen – and that does require an impartial British government.”

There are those who might point out that Sir John, when he was prime minister in the 1990s, put peace talks at risk by his dialogue with the Ulster Unionist Party, then the largest party in Northern Ireland. He had taken some brave decisions to open talks with both sides in Northern Ireland, but they stalled and the IRA ceasefire ended – partly because he was also talking to the unionists in order to shore up his position in the House of Commons, where he lost his majority through by-elections in December 1996.

However, that also means that Sir John knows exactly what he is talking about. The same applies to Jonathan Powell, who as chief of staff to Tony Blair was one of the architects of the final stages that led to the Belfast Agreement, after Sir John’s government was replaced. Mr Powell has called the deal with the DUP “a terrible mistake with lasting consequences”.

Mr Blair and Mr Powell have themselves been criticised for having boosted the DUP in the first place. They have been accused of sacrificing the UUP and SDLP, the centre ground of Northern Irish politics, for the polarised extremes of the DUP and Sinn Fein. But that was a dynamic that was unleashed unwittingly by the Belfast Agreement itself. Again, it means Mr Powell knows of what he speaks.

When Sir John and Mr Powell warn about the effects of Westminster opportunism on the “fragile” politics of Northern Ireland, where devolution has already been suspended because of the failure of the DUP and Sinn Fein to agree, we should listen to them.

Ms May no doubt feels that she has no choice if her government is to carry on. But she must remember that the national interest comes before her party’s interest.