British Finance Minister Sajid Javid resigned on Thursday, in a shock move that deals a blow to Prime Minister Boris Johnson's government just weeks after Brexit and a month before the annual budget.
The chancellor of the exchequer stood down after Johnson tried to use a reorganisation of his cabinet to get rid of some of Javid's aides, a source close to the departing minister said.
He was immediately replaced by senior Treasury official Rishi Sunak.
Javid's departure is a major upset at a turbulent time for Britain, just weeks after leaving the European Union on January 31, a process that has left the country with an uncertain future.
Johnson was on Thursday carrying out a cabinet reshuffle, his first since winning a comfortable parliamentary majority in the December general election.
Javid, a former City of London banker born into a working class Muslim family, was considered safe in his job despite reports of tensions between him and Johnson's senior aide, Dominic Cummings.
But rumours began to circulate after his meeting with the prime minister went on longer than expected.
"He has turned down the job of chancellor of the exchequer," a source close to Javid said.
"The prime minister said he had to fire all his special advisers and replace them with Number 10 special advisers to make it one team.
"The chancellor said no self-respecting minister would accept those terms."
Earlier, Johnson sacked his Northern Ireland minister despite his role in restoring devolved government to Belfast after a three-year suspension last month.
Julian Smith had helped end the political vacuum in Northern Ireland by persuading the two main parties to return to a power-sharing government last month.
The British province had no government since Sinn Fein and the Democratic Unionist Party (DUP) fell out in January 2017 over a renewable energy scandal.
Irish Taoiseach Leo Varadkar led tributes to Smith's efforts, saying he was "one of Britain's finest politicians of our time".
But The Times newspaper said Johnson felt "blindsided" by the deal because it includes an investigation into alleged crimes by British soldiers during decades of sectarian violence known as The Troubles.
Irish Foreign Minister Simon Coveney, who was also closely involved in the power-sharing talks, said that without Smith's leadership there would be no government in Belfast.
"You have been such an effective secretary of state for Northern Ireland at a time of real challenge and risk," he wrote on Twitter.
Arlene Foster, who as DUP leader was restored as first minister of Northern Ireland under the deal, also hailed his "dedication to the role".
Smith said it had been "the biggest privilege", adding that he was "extremely grateful" to Johnson for giving him the chance to serve.
Business Secretary Andrea Leadsom, Environment Secretary Theresa Villiers and Attorney General Geoffrey Cox were also shown the door.
But Foreign Minister Dominic Raab and Michael Gove, Johnson's de factor deputy, are staying.
Johnson held off from carrying out a cabinet reshuffle immediately after his December victory, choosing to wait until Britain left the EU.
After years of political turmoil over Brexit, he wants to focus this year on domestic issues, including investments in police, healthcare and infrastructure.
Johnson also named former aid minister Alok Sharma as the new minister in charge of the COP 26 UN climate summit, which is being held in Glasgow in November.
The former president, Claire O'Neill, was sacked last month. She responded with a blistering attack on his leadership, and warned that planning for the summit was "way off track".