The appointment comes after Ben Wallace revealed last month he would be leaving the role the next time Rishi Sunak made changes to his cabinet - as well as stepping down as an MP at the next election.

Mr Shapps had been serving as the energy security and net zero secretary before his promotion to the Ministry of Defence.

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The new post will be his fifth cabinet position in a year - having been transport secretary under Boris Johnson, undertaking a brief stint as home secretary under Liz Truss, before being appointed business secretary when Mr Sunak first took office.

Minister for children, Claire Coutinho, will take over the energy security brief - the first cabinet post for the MP who was only elected in 2019 - and her Conservative colleague David Johnston will fill her former post at the Department for Education.

Sky News' deputy political editor Sam Coates said both Mr Shapps and Ms Coutinho were "loyalists", adding they had been chosen by the prime minister "not just perhaps because of their skills, but because they have stuck by Rishi Sunak".

The new defence secretary wrote on X, formerly known as Twitter, that he was "honoured", and he paid tribute to his predecessor's "enormous contribution... to UK defence and global security".

Mr Shapps added: "As I get to work... I am looking forward to working with the brave men and women of our Armed Forces who defend our nation's security. And continuing the UK's support for Ukraine in their fight against Putin's barbaric invasion."

He garnered the support of some of his colleagues, with Conservative Leader of the Commons Penny Mordaunt saying he would do "a great job".

But the new hire has already attracted criticism from opposition parties, with Liberal Democrat defence spokesperson Richard Foord saying: "At a time when the Armed Forces need someone to stand up for them, Rishi Sunak has appointed a yes-man.

"The Conservative government merry-go-round has to stop. They have taken the Armed Forces for granted for too long, and we are all left less safe as a result."

Labour's shadow defence secretary John Healey congratulated his new opposite number on X, but added: "After 13 years of Tory defence failures, a change at the top will not change this record."

His colleague, shadow attorney general Emily Thornberry, also said: "It really doesn't matter how many new jobs Rishi Sunak gives to his friends. It's still just moving the deckchairs, they are still a sinking ship.

"This country really needs change and the only change we are going to get is if we get a Labour government."

Read more:
Who is Shapps? The defence sec whose appointment has ruffled feathers
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Shapps 'knows very little about defence'

Doubts have also been voiced from the sector itself, with former chief of the general staff of the British Army, Lord Dannatt, telling Sky News that Mr Shapps knew "very little" about defence.

He said Mr Wallace "did a good job, but he leaves with work in progress", adding: "Now we have a new defence secretary who knows very little about defence, and it's a complex portfolio. It will take him quite some time to get up to speed."

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Lord Dannatt added: "Although he may well have been appointed as someone who is going to support the prime minister and help the Conservative Party in its general election campaign, [defence chiefs] will be hoping that he will really understand defence and push the case for defence, not just for the Ministry of Defence's own benefit, but for the benefit of the whole country."

Mr Wallace officially resigned his post on Thursday morning, saying it had been a "privilege" to serve in the post for four years.

In his letter to the PM, he said the Ministry of Defence was now "more modern, better funded and more confident than the organisation [he] took over in 2019", and it was "back on the path to being once again world class with world class people".

And in a parting shot to secure his department's future, Mr Wallace added: "I know you agree with me that we must not return to the days where defence was viewed as a discretionary spend by government and savings were achieved by hollowing out.

"I genuinely believe that over the next decade the world will get more insecure and more unstable. We both share the belief that now is the time to invest."

Mr Sunak responded to the letter by saying the outgoing defence secretary had "served our country with distinction".

He added: "I fully understand your desire to step down after eight years of exacting ministerial duties. As you say, the jobs you have done have required you to be available on a continuous basis.

"But I know you have more to offer public life both here and internationally. You leave office with my thanks and respect."

Ukraine's minister of defence, Oleksii Reznikov, praised Mr Wallace too, saying: "His energy and dedication have allowed the boldest plans to be realised, and have enabled critically-needed resources to be mobilised at the right moments.

"Thank you, Ben! Together to victory!"

A Labour source said Mr Wallace had "shown important leadership in supporting Ukraine from the start".

But, while they said the former minister "deserves credit for his support" of Ukrainian fighters, "his record on British Armed Forces has been poor, with cuts and procurement failures the order of the day".

The source added: "The new defence secretary needs to get a grip, boost British forces and give them the resources, kit and accommodation they deserve."

'Homegrown' power

Ms Coutinho also said on social media she was "delighted" with her promotion.

"I will work with the prime minister to safeguard our energy security, reduce bills for families, and build cleaner, cheaper, homegrown energy," she added.

But, while congratulating her on the appointment, Labour's shadow energy secretary, Ed Miliband, said it "speaks volumes about the failures of Conservative energy policy that we are now onto the sixth energy secretary in less than four years".

He added: "The new energy secretary needs to recognise that Grant Shapps' approach has been a disaster and distance herself from it.

"His opposition to clean, homegrown power has damaged Britain's energy security, alarmed business and driven away investors."