Rishi Sunak described himself as a “serious candidate for serious times” as he kickstarted his campaign to become the UK’s next prime minister, with as many as 15 Conservative MPs expected to run for the party’s leadership. The former chancellor launched his bid on Friday after telling colleagues he had 80 to 100 MPs behind him. Some began declaring their support publicly, including Mark Harper, the former chief whip, Oliver Dowden, the former party co-chair, and Mark Spencer, the leader of the House of Commons. Sunak’s announcement came in a slickly edited three-minute video message posted on Twitter under the slogan “Ready for Rishi”. “I’m standing to be the next leader of the Conservative party and your prime minister,” he said. “Let’s restore trust, rebuild the economy and reunite the country.” His declaration capped an extraordinary week in Westminster in which Boris Johnson’s authority dramatically collapsed, with more than 50 members of his government quitting before the prime minister finally announced his resignation on Thursday. Saying the country faced “serious challenges”, Sunak strongly indicated he would resist calls for widespread tax cuts or a spending spree. “Do we confront this moment with honesty, seriousness and determination, or do we tell ourselves comforting fairytales that might make us feel better in the moment but will make our children worse off tomorrow?” he said. Tax cuts are expected to feature heavily in what is likely to be a chaotic and at times bitter leadership race. Another declared candidate, the one nation Tory Tom Tugendhat, has already called for the recent national insurance increase to be reversed. The former equalities minister Kemi Badenoch, announcing her candidacy in the Times, also appeared to back lower taxes, saying she wanted a “limited government focused on the essentials”. His announcement came as allies of Priti Patel, the home secretary, said she was seriously considering running, suggesting the politics of some of the other frontrunners were unclear. “You don’t need to think about Priti: you know where she stands,” one said, comparing her to Margaret Thatcher. The culture secretary, Nadine Dorries, a diehard Johnson loyalist, is also reportedly consulting colleagues about whether she has sufficient support to run. The defence secretary, Ben Wallace, considered a potential frontrunner, could also declare his bid. Arch Brexiter Steve Baker has announced that he would not run but would support a bid by the attorney general, Suella Braverman. On Friday evening, he tweeted: “I considered standing for the leadership. My priorities were delivering against our manifesto with our mandate, cutting taxes and seeing through Brexit. Happily I no longer need to stand. Suella Braverman will deliver these priorities and more.” Johnson fired the starting gun on the race on Thursday when he reluctantly quit as Conservative leader with a defiant speech describing his colleagues’ decision to oust him as “eccentric” and the result of “herd instinct”. One senior Tory with knowledge of Johnson’s mood described him as “confused” on Friday. “Not confused in a dumb sense – he fully understands the process. But he is confused as to their plan and strategy because there isn’t a clear way through.” Johnson continued to fill the posts in his caretaker government, with loyalist Peter Bone made deputy leader of the House of Commons and his policy chief, Andrew Griffith, a junior trade minister. Other cabinet ministers are expected to declare their candidacy in the coming days, including the new chancellor, Nadhim Zahawi, the foreign secretary, Liz Truss, and health secretaries-turned-backbenchers Sajid Javid and Jeremy Hunt. Zahawi is believed to have the most organised campaign, with a team in place advised by Lynton Crosby’s former business partner Mark Fullbrook. The executive of the backbench 1922 Committee will be re-elected next Monday, and is then expected to set out the rules and duration of the leadership race, including spending limits. While some MPs have been pressing for Johnson to step down as soon as possible, others are keen to see a full-blown contest, perhaps lasting until September, to allow for the backgrounds of the candidates to be properly scrutinised. Johnson’s premiership unravelled after it emerged that he appointed Chris Pincher as deputy chief whip despite knowing of his past alleged sexual misconduct. “I think all of the candidates have got significant due diligence vulnerabilities – things they are exposed on that will hit the media,” said one senior Conservative, adding: “There’s really going to be a big field – I think there could be 15 candidates or more.” A former cabinet minister said: “The worst possible thing that could happen to the Tory party is that they appoint somebody in a hurry, and that person has another ethical problem, whether it’s a money problem, a historic drugs problem, or a sex problem. If that happens, however good the policies are, we’re buggered at the next election.” Party insiders are warning that the race – in which the field will be narrowed down to two MPs, ideally by the time parliament rises on 21 July, before Tory members get the final vote – is expected to be hard-fought. “I think it will be a very divisive contest knowing the sort of people who are involved. I think there will be a lot of really nasty briefing,” said one senior Tory. Speculation at Westminster has also already turned to whether a new leader would feel obliged to call a snap election – perhaps within six months – to give them a personal mandate. “If they want to do anything big and new, it’s a 75 majority on the old manifesto and the Boris Johnson platform, which is dead,” said a party insider. “If you actually want to do stuff – radical things in a recession – then you have to put it to the country.” The Labour leader, Keir Starmer, has made clear he will push for a general election. At a press conference after the news that he would not be fined for an alleged breach of lockdown rules, he said: “If there’s a general election, this government will fall and we’ve got a plan for the country.” As the leadership race unfolds, there are likely to be perils and controversies ahead for Johnson’s caretaker role in No 10. While it has been agreed he will not introduce new policies or major fiscal decisions, Johnson’s spokesperson stressed that potentially contentious actions on existing plans, such as defending court cases to deport asylum seekers to Rwanda, and pushing through the bill to unilaterally amend the Northern Ireland protocol, would go ahead. Among other tricky moves looming is the decision on a series of public sector pay awards amid the threat of strikes. The government’s verdict on these could come as early as next week, potentially with several awards announced simultaneously.