Boris Johnson is dangling the threat of a rebellion over Rishi Sunak after a new post-Brexit deal was announced that will rip up the former prime minister’s protocol on Northern Ireland and ditch his legislation to override it. Although most Conservative MPs warmly welcomed the breakthrough after two years of negotiations, Johnson stayed away from the House of Commons chamber and is said not to have made up his mind about whether to endorse or oppose the “Windsor framework”. A source close to him said he was studying and reflecting on the government’s proposals. They did not deny that Johnson had urged the Democratic Unionist party (DUP), who hardline Tory Brexiters on the European Research Group have said they are in “lockstep” with, to think carefully before passing judgment on the deal. The source said they would not comment on private discussions, after PoliticsHome reported that he urged the DUP to be cautious amid suggestions it was prepared to endorse the agreement. While no Tory MPs have yet openly criticised the deal, the veteran Brexiter Bill Cash warned Sunak he would scrutinise the text closely before deciding what to do. “The devil, as ever, lies in the detail,” he said. Mark Francois, chair of the ERG, also said he hoped “we won’t find any nasty surprises which would materially undermine the position of Northern Ireland”. The ERG is expected to hold a full meeting for members on Tuesday night to decide how to respond to the Windsor framework, with a “star chamber” of lawyers assembled to scrutinise the plans for a veto for Stormont on new EU laws in Northern Ireland. Sunak vowed that MPs would get a vote on his deal “at the appropriate time”, and added the result “will be respected”. Several members of the ERG privately said they were broadly supportive of Sunak’s deal. “Provided the details live up to the press conference, fundamentally, I think this sounds like something they should be able to live with,” said one. Another said they believed only 10 or so “headbangers” were “prepared to let the perfect be the enemy of the good”. Some of the old Brexit “Spartans” who helped bring down Theresa May over her deal in 2019 are now part of the government, including Steve Baker. He welcomed the deal and said other pragmatists should too. However, the former culture secretary Nadine Dorries hit out at Baker for “gushing about the deal”, claiming he was a “key agitator” who helped to remove Johnson from Downing Street last July. She said: “What shred of credibility he has left would be destroyed if he came out against Sunak. He has nowhere else to go other than to grin and support.” Johnson has urged Sunak not to drop his protocol bill, which drew a legal challenge from the EU. But the prime minister is facing pressure to do so from senior European leaders, including from the French president, Emmanuel Macron, with whom he is expected to meet to discuss measures to tackle people being smuggled across the Channel in small boats. Any rebellion may end up being small, Tory strategists believe. Hardline Brexiters, including the UK’s former negotiator David Frost and ex-business secretary Jacob Rees-Mogg, have so far refrained from making critical interventions about the state of the deal from reports over the past week. But even a dozen Conservative MPs opposing the deal could trigger bigger problems for Sunak further down the line. Anand Menon, the director of the UK in a Changing Europe thinktank, said: “The danger for the prime minister is that opposition might be cumulative. A few rebels on the protocol, a few more on the budget – this could all build into a real headache should the May local elections go badly.”