Keir Starmer’s closest aides are “war-gaming” how to win a leadership contest ahead of Andy Burnham’s much-anticipated return to Westminster if he wins the Makerfield byelection, the Guardian understands.

Downing Street sources said the prime minister had taken the last fortnight to think seriously about his future but was now “hellbent” on fighting any contest. His team is working through various scenarios, including sacking ministers who publicly supported Burnham.

However, several close allies acknowledged that Starmer’s future may be out of his hands. “Keir may be determined to fight any challenge. But when it comes to it, he might not have a choice. As one former prime minister put it, when the herd moves, it moves,” a loyalist minister said.

Starmer was self-aware enough to stand aside should he get closer to the general election and feel that Labour could not win against Reform with him at the helm, but could with somebody else, they said, but added: “He’s not there yet”.

Burnham said last week that he would seek to join any potential Labour leadership contest should he beat Reform in the byelection next week, and return to Westminster, but has not ruled out challenging Starmer himself if it should come to it.

So the prime minister’s team – despite a degree of scepticism in its ranks – is readying itself. One senior aide said: “Maybe it is over. But if so, Keir’s not going without a fight. Our plan is to be more Harry Truman and less Joe Biden.”

Steve Reed, one of the cabinet ministers most loyal to Starmer, said he was convinced the prime minister would fight any leadership contest, and should even offer Burnham a job should he win in Makerfield.

“I know how he feels, because we speak to each other, and he is determined to deliver the change that he was elected to deliver,” Reed said.

“He led us to one of the biggest landslide victories in our party’s history [four years after becoming leader]. A man that can deliver that kind of change in that period of time is not a man that lacks determination.”

Allies of the prime minister argue that Burnham is unprepared for a leadership challenge, as he and his team have been juggling the Makerfield campaign alongside planning for what might follow.

They believe that in the first few weeks after a byelection victory, much of his time will be spent campaigning in the subsequent election for a new mayor of Greater Manchester.

Once that has happened, MPs go away for the summer and then party conference season begins. “By the time you get there, Burnham’s momentum has started to ebb,” said one.

They also said Starmer will not fold at the first sight of a challenge. “You don’t abandon your house just because someone shakes a stick outside it,” said one.

Starmer was still making the same arguments about the damage inflicted by a contest as he did when Wes Streeting attempted his coup, sources said, telling MPs it would paralyse the government’s ability to get things done.

During meetings with groups of MPs, he has said of the perils of the Labour party turning inwards. “He thinks that if we lower the threshold at which we can depose PMs then we’re just like the Tories,” one said.

Polling of party members has given his supporters some hope: in a YouGov poll, 59% say they would back Burnham against Starmer, with 37% saying they would vote to keep the prime minister in place. Allies believe the prime minister could secure a 10% swing once Burnham faces scrutiny.

They hope that once the limelight turns on to Burnham, more MPs will conclude that he would struggle as prime minister. “The more Andy is tested, the more people see he’s not up to the job,” one said. Others cited his BBC Newsnight interview, arguing he was out of his depth.

But Starmer presides over a divided and restive party. After Labour’s devastating local election results last month, several cabinet ministers urged him to think about his future.

No 10 insiders admit that if they – and others – now publicly call on the prime minister to go, it would be difficult for him to resist the pressure.

Despite Downing Street’s on the record denial he is planning to tell ministers to quit if they back Burnham in any contest, sources said Starmer would not hold back.

“There is a world where he will get there first and sack people rather than see them resign. Rachel, Ed, Shabana being the key ones. He has shown he has no qualms about sacking people when he needs to,” an insider said.

He has already alarmed ministers with the prospect of cuts to departmental capital budgets, to pay for an increase in defence spending, which one described as a “scorched earth” strategy.

“He’d be completely trashing his legacy, and his growth strategy, if he cuts infrastructure spending. The only thing he’d be remembered for is broken promises,” one cabinet minister said.

With Starmer also due to implement a social media ban for under 16s in the coming days, his critics in the party have suggested that he is trying to secure his legacy in power. Downing Street insiders said the “L-word” was banned.

Some allies have indicated he could reach some sort of accommodation with Burnham, with one saying he could be offered a job if he returns to Westminster. “He’s a very talented politician, a great communicator. There’s a very big role for Andy as part of this government,” they said.

But others said there were no channels of communication between the pair “because there is no trust whatsoever”, and that Starmer was now highly unlikely to campaign for Burnham in the byelection. “Of course that may be a relief for Andy,” one source added.

Even Starmer’s closest allies acknowledge that, when it comes to it, the decision may end up being made for him. “I think people underestimate how much agency he has in this,” one said. “He’s in angry denial,” another added.

Burnham’s allies also believe the prime minister is struggling to face the reality that there are enough supportive MPs to man the barricades with him.

“He might be increasingly defiant, but it is like a dog barking in an empty room,” said one. “Eventually its owner is going to come back home and kick it out.”