Scott Morrison has moved Linda Reynolds into the cabinet in an effort to settle the government after another two high-profile departures, with confirmation that Christopher Pyne and Steve Ciobo won’t contest the next federal election.

The prime minister announced on Saturday that Reynolds, a Western Australian senator, would be promoted to cabinet, replacing Ciobo, who is currently the defence industry minister, with the Queenslander stepping down from the ministry.

Pyne, currently defence minister, and a key moderate powerbroker within the government who delivered Morrison the Liberal leadership last year, will remain in his portfolio until the election. Morrison said on Saturday that Reynolds, a former army brigadier, would be the defence minister if the Coalition won the election.

The departures are a blow for Morrison, fuelling public perceptions that senior figures have now written off the government’s chances of retaining office in May. Pyne and Ciobo’s departures follow recent decisions by Kelly O’Dwyer, Julie Bishop, Michael Keenan and Nigel Scullion to bow out of politics.

Government MPs have also been concerned that Craig Laundy, who voiced his frustrations when Malcolm Turnbull was deposed as prime minister last year, will also bow out, creating problems for the government in the Sydney seat of Reid, which Labor has in its sights.

In a statement issued by his office, Morrison said it was the responsible course to leave Pyne where he was, given in the area of defence, the government was “currently managing a series of important strategic issues”.

“This will ensure a consistency in our approach and the opportunity for a seamless handover to minister Reynolds, should we be successful at the election,” the prime minister said.

“As a cabinet minister in the defence portfolio minister Reynolds will also have a unique opportunity to transfer into the role in the event the government is re-elected.

“Linda has the talent, the experience, the knowledge and the determination to get the job done. Linda has demonstrated this with the responsibilities I gave her last August leading our disaster response from North Queensland to Tasmania.”

“She has worked closely and extensively with senior ADF personnel in her role as a brigadier. This will also assist her be successful in the role.”

Reynolds said she felt well prepared for the role given her background. “I am enormously grateful for this opportunity.

“I understand the challenges we confront regionally and globally,” she said. “I know what it takes to ensure the men and women who serve our nation in the ADF have the best possible equipment – and wherever possible it is made here in Australia.”

There have been persistent rumours about Pyne and Ciobo circulating since the beginning of the year, but neither confirmed their intentions until the weekend.

Ciobo backed Peter Dutton in last year’s leadership battle, and was moved out of the trade portfolio in the fallout, remaining in cabinet, but effectively as Pyne’s junior in the portfolio.

Pyne, a longtime Turnbull supporter, was promoted to the senior defence portfolio after throwing his numbers behind Morrison, viewing him as a better prime minister for the moderate faction.

At a news conference on Saturday, Pyne said he was grateful to be quitting on his own terms.

“It’s nice to go when people are asking you to stay, rather than when they’re trying to plot to remove you,” he said.

He dismissed suggestions that he was quitting because of the government’s prospects, saying he believed the Coalition would win the election and retain his seat of Sturt.

“Being in politics is not a life sentence and I’ve been there for 26 years,” he said.

“When you get to that point where you decide it’s time to go, it would be running under false pretences for me to ask the voters of Sturt in May to re-elect me.”

Pyne – a veteran since the Howard era – has recently reflected on the strains of parliamentary life, and on the impact of last year’s leadership implosion.

He had told the Sydney Morning Herald Turnbull was “the kind of person that should have been prime minister of Australia: urbane, highly intellectual, successful, broad, visionary, clever, articulate, funny, charming, everything that a modern leader and a modern prime minister should be”.

“And I found it very disappointing that too many of my colleagues didn’t see in Malcolm what I saw and still see in Malcolm.”

But on Saturday he said would have retired even if Turnbull had remained prime minister.