The UK defence secretary, Ben Wallace, and one of his deputies, James Heappey, have indicated they are prepared to quit their jobs if Liz Truss does not honour her campaign pledge to spend 3% of GDP on defence by 2030. The beleaguered prime minister’s commitment has appeared under threat following the surprise appointment of Jeremy Hunt as chancellor on Friday. Since then, Hunt has said no department would be immune from spending cuts. But on Tuesday morning a defence source insisted that Wallace, one of the most highly regarded ministers in the government, “will hold the prime minister to the pledges made”, prompting fresh speculation he may quit if the pledge is not met. The source, referring to the crisis in Ukraine, added that “current world events, and allies commitments show that defence is not a ‘discretionary’ spend but a priority”. Wallace is working on the basis of the revised spending plans, in which budgets will increase from 2.1% of GDP to 2.5% by 2026 and 3% by the end of the decade to total an estimated £100bn a year. It reflects a promise “the prime minister made clear in her leadership campaign” the source added, and, it is understood, further undertakings given by Truss in a letter of appointment setting out her priorities for Wallace on 29 September. Earlier, Heappey, the armed forces minister, had said he would also resign if the 3% commitment was dropped. When asked on LBC by presenter Nick Ferrari “would you quit?” in those circumstances, Heappey replied: “Yeah, but no one said, Nick, that 3% is not going to happen by 2030.” Wallace is on an hastily arranged trip to Washington, where he will meet his US counterpart, Lloyd Austin, and other senior White House figures to discuss the crisis in Ukraine and other “shared security concerns”. The cabinet minister had been due to give evidence to the defence committee on Tuesday afternoon, but the hearing was called off at short notice, prompting speculation about the nature of the trip. Heappey had said, cryptically, that Wallace’s discussions were significant. Wallace was going “to have the sort of conversations that ... [it’s] beyond belief really, the fact we are at a time when these sort of conversations are necessary,” he told Sky News. Previously Hunt had said the full reversal of Liz Truss’s economic plans would mean “decisions of eye-watering difficulty”, and suggested nothing was off the table, including defence and health spending and the pensions triple-lock. On the 3% pledge, Heappey also told Sky News: “The commitment the prime minister made is 3% by 2030 and to be clear like the secretary of state [for defence, Ben Wallace] that’s something that I believe must be delivered given the need to keep our nation safe given increasingly uncertain times.” But he conceded there may need to be some short-term budget adjustments, saying: “If in the very immediate term there is a requirement to look at what we can do to help the Treasury out that’s a discussion for the chancellor to have with the secretary of state.” With Truss’s future in grave doubt over the scale of her errors that fuelled market turmoil, Heappey said there was no candidate who could replace her who could unite the Conservative party. “I think the vast majority of colleagues recognise that a mistake was made. The prime minister has owned that and apologised for it. And is the alternative to rowing in behind the prime minister and making success of her government is to throw ourselves into another period of great rancour?” he told the BBC Radio 4’s Today programme. Asked by Sky News how many more errors Truss could make, he said: “I suspect given how skittish our politics are at the moment, not very many.” Pressed how many, he said: “I don’t think there’s the opportunity to make any more mistakes.” The shadow chancellor, Rachel Reeves, told the Today programme that the government was embarking on a programme of austerity that had never been part of the Tory mandate. “The latest chancellor, the fourth in four months, was a key architect in austerity season one, and he’s now saying that what we need is austerity season two,” she said.