In a dramatic intervention, the former Conservative prime minister warned the peace process is “fragile” and could fall part if the British government is no longer seen as “impartial”.
“People shouldn’t regard it as a given. It’s not certain, it’s under stress, it’s fragile,” Sir John said.
He urged the Prime Minister to pursue a minority government without “the baggage” of an agreement with the DUP.
And he highlighted the danger of “hard men still there, lurking in the corners of communities deciding they wish to return to some sort of violence”.
“We need to do everything we conceivably can to ensure that doesn’t happen – and that does require an impartial British government,” Sir John said.
While peace in Northern Ireland was not in danger of collapsing imminently, it could “unwind”. “That does worry me a great deal about the peace process,” he told BBC Radio 4.
The warning came just minutes after Arlene Foster, the DUP leader, and her deputy Nigel Dodds, arrived at No 10 for talks with the Prime Minister.
It is expected to lead to a “confidence and supply agreement”, to give the Tories a 13-seat Commons majority with support on confidence motions and key budget votes.
But Sir John said: “I understand why she wishes to shore up her Parliamentary position. That’s entirely understandable and I sympathise - but my main concern is the peace process.”
“We need to be prepared for the unexpected. We need to hope for the best – but prepare for the worst,” he added.
Sir John also warned that voters in the rest of the UK would view the deal – and the inevitable extra funding to that would flow to Northern Ireland – as “cash for votes” and feel a “grievance”.
“How is that going to be received in Wales, or in Scotland or amongst the 'just-about-managings' everywhere across the UK?” he asked.
“They would see it as the Government paying cash for votes in Parliament. And, in doing so, I think that could well cost votes in the country for the Conservative party at a subsequent election.”
Sir John also warned the Prime Minister that her hard Brexit stance was “increasingly unsustainable”, with the loss of her Commons majority.
He urged her to try to “negotiate a better deal on free movement” and on the EU single market, by being “more generous on immigration”.
“A hard Brexit was not endorsed by the electorate,” Sir John said.