A new Labour prime minister with a landslide majority faced the first real test of his authority with a party rebellion from MPs concerned about a tough stance on welfare.

That prime minister was Tony Blair, in December 1997, when 47 Labour MPs voted against the government including a frontbench resignation over a bill to cut benefits for single-parent families. But no one lost the whip.

On Tuesday evening, 27 years later, that was the crucial difference – seven Labour MPs who voted for a Scottish National party (SNP) amendment to end the two-child benefit limit were suspended from the party.

Keir Starmer’s decision was a show of force that was unprecedented for a new government on an opposition amendment as well as being entirely in keeping with his ruthless mode of operating.

But it still came as a shock when the new prime minister, as he has done time and again, took the harshest possible approach to party discipline. He has suspended the whip more readily than any Labour leader, though his place in history on that front is rivalled by Boris Johnson for the Conservatives.

Still there had lingered a belief that, somehow, this time, he would take a different approach.

On Tuesday afternoon, a rumour went round Westminster that the chief whip, Alan Campbell, had directly told at least one possible rebel that they would lose the whip by voting for the SNP amendment.

Unlike many recent rebellions, this one was barely organised – there was no shadow whipping operation and little coordination between the MPs. But those who saw Campbell personally were left in no doubt about the consequence if they rebelled, and the news began to spread.

Earlier in the afternoon, the threatened rebellion had been much larger – 19 had signed an amendment from the Labour MP Kim Johnson that was not selected but made an identical request. There were others who had criticised the decision not to scrap the cap, and not just those from the left of the party. Two outspoken critics on the policy and its cruelty were Rosie Duffield and Stella Creasy.

“They suddenly upped the ante,” one MP said of Labour’s whipping operation: “It seemed a bizarre gear-change when all the previous mood music had been that they were considering [dropping the benefit cap].”

There were attempts at overtures and persuasion. Liz Kendall, the work and pensions secretary, held a briefing, though a hastily rearranged time meant few MPs could attend. Both she and the employment minister, Alison McGovern, reached some MPs directly – including condemning briefed language over the weekend that the government saw this issue as a “virility test” to show how it would defy internal dissent.

Among the moderate wing of the party, where some MPs were tempted to rebel on a point of conscience, they were deeply shocked that the party would use the threat of suspension.

Many had believed they were being given a tacit nod and a wink by Starmer and the education secretary, Bridget Phillipson, on Monday that change was coming at some point via the new child poverty taskforce.

But those are mixed messages to say the least, when there is no inch being given by the Treasury. Dropping the cap is estimated to cost £1.7bn a year, but experts say it would be the most cost-effective way of immediately reducing child poverty – lifting 300,000 children above the breadline and pulling 700,000 more out of extreme poverty.

It is clear the rebellion would have been much bigger had it not been for the nuclear threat. And there had already been a divide among MPs who back scrapping the cap about whether the rebellion was organised too hastily, leaving the dissenters with nowhere else to go.

Among some on the left, there had been a feeling that Starmer’s advisers might be rethinking their disciplinarian approach given MPs were still at risk in some constituencies of votes being eaten by independent candidates or Greens.

One said they believed Starmer would see it as beneath him – petty, even – as a prime minister having recently spent an afternoon with Joe Biden to then be concerning himself with how a few ex-Corbynite shadow ministers voted on an opposition amendment. “If you’re too thin-skinned, as prime minister of the UK, don’t you look weak?” asked one. “We expected it to barely be on his mind.”

Now Starmer has made it clear that is not his instinct. In fact, with a big and ambitious new parliamentary party, first-time MPs have had the fear of God put into them if any are tempted to try to make their mark in this manner. “What message does it send to newbie rebels if you don’t follow through?” one Labour source said.

But such apparent machismo can backfire. There have already been electoral consequences, with at least eight seats affected by a strict Labour approach – from the alienation of Muslim voters to the rise of the Greens.

And it still has the potential to do more damage. In Westminster, gleeful SNP MPs have been littering the corridors with copies of a newspaper front page where the former prime minister Gordon Brown promised to end child poverty.

And in the immediate aftermath of the vote on Tuesday, the Green deputy leader, Zack Polanski, targeted his attack directly at Stella Creasy in Walthamstow, an east London seat he is thought to covet. “At the election in Walthamstow, Labour went down 16% and [Greens] went up 16%. There’s 40 places where Greens are in 2nd place to Labour and we’ll see you at the ballot box,” he tweeted.

Those who defend Starmer’s approach come back to the same argument – that the election was not only a vote for change but to end the chaos of Conservative rule where MPs seemed to need their fix of rebellions and plots on a weekly basis. This approach is designed to ensure none of their new recruits get any taste for that.