Rishi Sunak has warned his warring MPs that the Conservatives must “unite or die” in the face of a “profound economic challenge” after winning the race to be the next prime minister.

The former chancellor ruled out opposition demands for a general election after winning the Tory leadership contest on Monday when rival Penny Mordaunt failed to win the support of MPs.

Liz Truss, who will succeed Mr Sunak less than two months after she beat him in the last contest, congratulated her soon-to-be successor and told him “you have my full support”.

Despite calls for unity, a Tory source told Politicalite last night that there was already “talk of dirty tricks and rebellions”.

“The same old party divisions remain,” the source said.

“They went after Boris’ supporters to try to stop him from getting to 100,” they said. A member of Sunac’s campaign team later admitted that they had spent the weekend trying to persuade MPs who had publicly declared their support for Johnson to side with the former chancellor.

“There are people who feel that they owe their careers to Boris and are afraid of losing their places,” they said. “They felt they had to publicly support him, but privately they said they would vote for Rishi.”

The deputy added: “Rishi actually turned to Boris and said – we have the numbers, but you don’t.” If you oppose me, we will destroy you.’

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With a victory on Diwali, Mr Sunak will become Britain’s first Hindu Prime Minister, the first of Asian heritage and the youngest in more than 200 years at the age of 42.

He will be formally appointed to a role in the handover under the king’s watch in the coming days as discussions take place between Ms Truss and Mr Sunak over a timeline.

Ms Mordaunt, the leader of the House of Commons, withdrew from the race to give Mr Sunak a spectacular political comeback as she failed to secure the 100 nominations from Tory MPs needed by 2pm.

With Boris Johnson abandoning his own bid to return, Mr Sunak will enter No 10 unopposed and avoid an online vote by Conservative members who rejected him for Ms Truss last month.

Mr Sunak hit the ground running, telling Conservative MPs behind closed doors in the House of Commons that they were facing an “existential moment”.

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Three MPs in the room said his message to the party was that they must “unite or die” as they focus on achieving community priorities during the cost of living crisis.

Labor has launched calls for a new general election as the Tories move to their third prime minister after the mandate won by Mr Johnson in 2019.

Senior Tory Simon Hoare said Mr Sunak had told the party he would not give in to the opposition’s demands, with the MP adding to reporters: “He’s actually going to go to ground. We have no time to lose.”

Mr Sunak now faces the daunting task of guiding the country through an economic crisis that has only been exacerbated by the chaotic legacy of Ms Truss, who was ousted after just six weeks in office.

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In an 86-second address to the nation, Mr Sunak pledged to serve “honestly and humbly”.

He praised Ms Truss for serving the nation with “dignity and grace” during “exceptionally difficult circumstances”.

He said he was “humbled and very honoured” to be chosen as the next prime minister, which he described as “the greatest privilege of my life”.

“The United Kingdom is a great country, but there is no doubt that we are facing a serious economic problem,” he added during the speech, in which he did not take questions from the media.

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“Now we need stability and unity, and I will make it my highest priority to unite our party and our country.”

The king was understood to be traveling to London on Monday afternoon from the private royal estate of Sandringham, but Downing Street confirmed the handover would wait another day.

In a statement posted online two minutes before the deadline, Ms Mordaunt said her opponent now had “full support”.

“These are unprecedented times. Despite the tight schedule of the leadership contest, it is clear that colleagues feel that we need certainty today,” she said.

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“They made this decision in good faith for the benefit of the country.”

Sir Graham Brady, who in 1922 was chairman of the Conservative Backbench Committee and managed the contest, said he had received only one valid nomination form.

“Therefore, Rishi Sunak has been elected as the next leader of the Conservative Party,” he added.

Sources in the Mordaunt camp said she had received 90 nominations, although the number of those who have publicly announced their support for her is much lower.

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Labor deputy leader Angela Rayner said: “Rishi Sunak has no mandate and no idea what working people need.

“We need a general election to give the public a say on Britain’s future – and a chance to start a new Labour.”

Westminster SNP leader Ian Blackford made the same demand and said his party would back Labor in a vote of no confidence if they passed it.

Affairs Minister Jacob Rees-Mogg, a Johnson supporter who once derided Mr Sunak as a “mourning socialist chancellor”, said he would “support his leadership” and called for “now is the time for party unity”.

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Conservative Party leader Sir Jake Berry called for an end to Tory infighting, saying: “Now is the time for the whole party to come together and unite four squares behind Rishi as he continues the vital work of tackling the challenges we face. as a country.

“The time for internal debate is truly over and I know that under Rishi Sunak’s leadership we can and will deliver on the priorities of the British people.”

James Cleverley, who backed Mr Johnson’s return to No 10 less than two months after he quit amid a series of scandals, called for an expected cabinet overhaul to include top ministers rather than focus on loyalists .

The Foreign Secretary told Sky News: “We have to have the first 15 on the field. I know Rishi understands that.’

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Mr Sunak’s transition from MP to Prime Minister was the fastest in modern political history, having first won the Richmond constituency in North Yorkshire in 2015.

His path to the top job has not been without problems, with him being fined alongside Mr Johnson for breaching coronavirus rules and facing questions about his wife Akshata Murthy’s ‘non-domestic’ status for tax purposes.