Rishi Sunak has been warned that more than 100 Tory MPs could rebel over a deal with the EU to help break the post-Brexit deadlock in Northern Ireland, as Boris Johnson launched a major intervention calling for him to take a tougher line with Brussels. Pressure is growing on the prime minister as government sources said tense talks in Downing Street over the weekend on overhauling the Northern Ireland protocol were yet to yield a breakthrough. They cautioned that the hoped-for timetable of an agreement being announced on Monday and a Commons vote on Tuesday was at risk of slipping. One insider said, “We’re ready to go,” but claimed that nervousness about opposition by the Democratic Unionist party (DUP) and Tory backbenchers was to blame for the hold-up. Ireland’s deputy prime minister, Micheál Martin, was flying to Brussels on Sunday night for a meeting with the European Commission vice-president, Maroš Šefčovič, according to reports on Irish broadcaster RTE. He was due to be at the European Commission for a meeting of the foreign affairs council before the first anniversary of Russia’s invasion of Ukraine. There was a backlash on Sunday over details leaking out about concessions said to have been made by the UK, and a demand by Brussels that Sunak axe a controversial bill designed to unilaterally rip up some Brexit arrangements in Northern Ireland. In the first pronouncement by Johnson on Brexit since he left office, a source close to him said he believed “it would be a great mistake” to drop the legislation and hinted that it should be maintained as leverage over the EU. James Duddridge, a former Brexit minister and close ally of the former prime minister, said talk of the European court of justice retaining a role would also be a “wedge” to fulfilling Brexit. “It won’t just be the so-called ‘Spartans’,” he told Sky News, referring to the nickname given to the few dozen diehard purists. “There will be a large number of Brexiters, possibly the majority of the parliamentary party, and potentially running into treble figures.” There is no formal requirement to hold a vote, but Downing Street is considering holding one given concerns that they may end up being forced to – and would win regardless due to Labour’s commitment to support any deal. Hours after Johnson’s intervention, other Tories quickly sided with him. David Frost, the UK’s former chief Brexit negotiator, stressed there was “no deadline” for talks on overhauling the protocol. He said Sunak should “push on with the protocol bill, so that our negotiators are in the strongest possible position”. Simon Clarke, the former levelling-up secretary, signalled that he would not support “anything that keeps Northern Ireland subject to EU law or in the single market” and said the protocol bill “remains a clean solution to ensure all parts of our country are treated equally”. Dozens of MPs in the hardline group of Brexiters known as the European Research Group are planning to meet on Tuesday to discuss details of any concessions said to have been made by the UK, the Guardian understands. Despite widespread annoyance at Johnson’s intervention among his critics in the party, Penny Mordaunt, the Commons leader, claimed it was “not entirely unhelpful”. She said, “Boris is being Boris” and hinted it was “helpful to remind” the EU about the controversial protocol bill. Though Mordaunt said there were “optimistic signs”, she added: “Both sides of the negotiations have said we’re not there yet.” But some Tories want Sunak to keep the legislation halted. Robert Buckland, a former justice secretary, told the Guardian there was a “a narrow supporting legal argument” for the bill when it was progressing through parliament last year. But he added: “Now that the negotiations are real and progressing, I think that the situation has significantly changed. The bill is a dead letter.” Others dismissed the pronouncements by Johnson on Brexit. George Osborne, the former chancellor, said Johnson was interested solely in “becoming prime minister again”. Osborne told Channel 4’s Andrew Neil Show: “He wants to bring down Sunak and he will use any instrument to do it … If the Northern Ireland negotiations are that instrument, he will pick it up and hit Sunak over the head with it.” Naomi Long, leader of the Alliance party, also accused Johnson of thinking about himself rather than the legacy of the protocol. “He created this mess – he needs to sit this one out,” she said. The DUP has kept quiet since its meeting with Sunak in Belfast on Friday, after which its leader warned that the plan proposed “currently falls short of what would be acceptable”. While Downing Street believes its approach meets the DUP’s seven tests, it fears a three-pronged attack from the party, Johnson’s backers and the ERG. One source abreast of the negotiations said No 10 was keeping talks as secret as possible because “there would be a feeding frenzy of piranhas on both sides as soon as anything got out, killing any chance of a deal”.