James Cleverly has warned against "bitter infighting" among the Tories and said his party needs to "get our act together" ahead of the race to replace Rishi Sunak as leader.
Mr Cleverly has been appointed to shadow his former role of home secretary in the former prime minister's interim shadow cabinet after Labour's landslide win saw the Conservatives net their worst ever election result.
The party needs to conduct a "sensible post mortem on what went wrong and finding the right path forwards", Mr Cleverly wrote in The Times.
He wrote: "As we do this we must remember two vital things. Firstly, it cannot descend into bitter infighting and finger pointing. That is exactly how we ended up here."
A narrower offer will not win back voters that have been lost to the left or right, Mr Cleverly warned.
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He said: "We must get our act together. We need to unite in order to deliver.
"It will take humility and hard work, to recover our reputation for competence and integrity, to rebuild trust in our party, and unite behind a broad platform that will give people a reason to vote Conservative again."
Right-wingers and more moderate Tories are expected to battle it out for the top job in a contest that could shape the party for years to come.
Mr Sunak last week announced he would step down as party leader once the formal arrangements for choosing a successor are in place.
If he throws his hat in the ring, Mr Cleverly could face competition from high-profile right-wingers Suella Braverman and Kemi Badenoch.
Dame Priti Patel, who served as home secretary between July 2019 and September 2022, Robert Jenrick, who previously served as immigration ministerand ex-health secretary Victoria Atkins could also put themselves forward.
Jeremy Hunt, a centrist figure in the party, has ruled out running.
Tory MPs usually vote to select the top two candidates to put forward as potential leaders, with the party membership choosing the ultimate winner.
However, the rules and timeline of the race would be set out by the backbench 1922 Committee, which has elected Bob Blackman as its new chair.
Ms Braverman meanwhile has hit out at "liberal Conservatives" at an event in Washington DC and at a Popular Conservatism event in London said that Reform UK is an "existential threat" to the Tories.
Ex-Tory MP Sir Jacob Rees-Mogg was also at the event and spoke of the need to win back voters from Reform, which won five seats, including Clacton in Essex for leader Nigel Farage.
Ms Braverman, appearing via a recorded video, said of the loss of her party's 80-seat majority from 2019: "We were going to stop the wave of illegal migrants landing on our shores. We were going to cut taxes.
"We were going to stop the lunatic woke virus working its way through the British state.
"The harsh reality - this is a lesson we all need to learn and face up to - is that we did none of that."
The Tories made a net loss of 251 seats at the election last week, leaving the party with 121 MPs.
Labour won a landslide with 412 MPs, ousting several senior Tory figures including Sir Jacob, former Commons leader Penny Mordaunt, defence secretary Grant Shapps and ex-prime minister Liz Truss.
She added: "Historically in British politics, we have had a monopoly on the right-wing vote.
"That's one of the reasons we have been so successful.
"The left, by contrast, for the last 100 years has been split.
"Left wing have had a choice of party to go for and we by contrast have had the luxury of a monopoly, but no longer, and that is why the Reform party presents an existential threat to us electorally."
