Rishi Sunak, the incoming prime minister, has told MPs that the Conservative party is facing an “existential threat”, in his first address to them on becoming party leader, after Penny Mordaunt failed to get 100 nominations to reach the threshold to trigger a ballot. In his speech, given behind closed doors to MPs, Sunak said the party should “unite or die” and vowed he would return the party to the values of its 2019 manifesto, which helped win the party an 80-seat majority. Ruling out an early general election, he said he would lead a government of serious Conservative values and make his first priority tackling the economic crisis. Sunak will become prime minister without MPs or members having voted for him, after Mordaunt pulled out of the race in an apparent acknowledgement that she had not reached the necessary 100 MP threshold to progress. Two minutes before the nomination process closed at 2pm, Mordaunt tweeted that she had pulled out and that Sunak had her “full support”. Sunak’s other rival, Boris Johnson, pulled out on Sunday night, despite claiming he had the support of 102 MPs. Five minutes after Mordaunt’s withdrawal, Sir Graham Brady, the chair of the Conservative backbench 1922 Committee, formally announced the result. “I can confirm that we have one valid nomination, and Rishi Sunak is elected as leader of the Conservative party,” he said. Sunak will formally take over as prime minister from Liz Truss after meeting the King at Buckingham Palace, most likely on Tuesday, at which point Truss will have served 50 days in the job. It is understood the King was travelling back to London from his Sandringham estate in Norfolk on Monday afternoon. Sunak, the former chancellor who came second in the leadership contest against Truss in the summer, had secured the backing of more than half the parliamentary party by Monday morning. After his victory, waiting MPs gave him a rapturous reception, with cheers and banging of desks. He told them the party faced “an existential threat”, with polls showing the weight of opposition to the Conservatives, and he said his focus would be on “policies and not personalities”. “He said we could not pretend the last few weeks and months had been easy or edifying or helpful,” said Simon Hoare, a longtime Sunak backer. “We are going back to serious, pragmatic traditions of Conservative government. The message we heard was about going forward – as a party and a government this is about the future and shaping the future. We cannot rewrite history. We will play the hand we have got, but it is not an inevitable threat we face.” Sunak told MPs that his ambition was to have a “highly productive UK economy”, stressing a commitment to levelling up and the pledges of the 2019 manifesto, and said he the party backed low taxation but that it had to be affordable and deliverable. He said a stable and productive economy would be the engine that drives a well-funded health and education service and delivers on net zero, and he said it would be an “environmentally focused government.” Sunak said there would be no early general election, though he acknowledged that opposition parties would inevitably clamour for one. He said he would ask the British people for space and time to resolve the problems the country is facing. There was no commitment to spending cuts, but Sunak said it would be a “tough period” for the government. Stressing a need for stability, MPs said they inferred that Sunak would ask Jeremy Hunt to stay on as chancellor. “Time is not on our side, we have no time to lose,” Hoare said. Iain Duncan Smith, a prominent backer of Johnson and Truss, said it was right that a decision had been made quickly. “There is no more messing around, it is time to end the psychodrama and get on with governing,” he said. Duncan Smith said it was time to end personality politics. “He [Sunak] knows now we have to deliver what we promise. His first priority is to stabilise the economy and get that moving, then all the other things we promise to do, make the most of Brexit, levelling up in left-behind areas.” Sunak, whose parents are of Punjabi Indian heritage, will be the first person of colour to become British prime minister. He is not the first minority ethnic PM – Benjamin Disraeli, who held the office twice between 1868 and 1880, was of Jewish heritage. At 42, Sunak is the youngest prime minister for more than 200 years. His victory, in effect a coronation with not a single formal vote being cast, even by MPs, has reignited calls for a general election, given he is the third Conservative PM since Boris Johnson won in 2019. Angela Rayner, Labour’s deputy leader, said: “The Tories have crowned Rishi Sunak as prime minister without him saying a single word about how he would run the country and without anyone having the chance to vote. Rishi Sunak has no mandate and no idea what working people need. We need a general election so the public get a say on the future of Britain – and the chance for a fresh start with Labour.” Ed Davey, the Liberal Democrat leader, said Tory MPs had “installed another out-of-touch prime minister with no plan to repair the damage and without giving the British people a say”. Mordaunt, who was reportedly stuck on about 90 nominations, said in a statement: “These are unprecedented times. Despite the compressed timetable for the leadership contest, it is clear that colleagues feel we need certainty today. They have taken this decision in good faith for the good of the country. “Members should know that this proposition has been fairly and thoroughly tested by the agreed 1922 [Committee] process. As a result, we have now chosen our next prime minister. This decision is an historic one and shows, once again, the diversity and talent of our party. Rishi has my full support.”