When Catherine West announced she would challenge Keir Starmer, she was labelled a stalking horse, the slightly arcane political slang for someone testing out a bid on behalf of others. A couple of days on, and some Labour MPs say the better equine analogy is a Grand National competitor that has shed its rider and bringing chaos to the race.

Some colleagues of the north London MP are even blunter in their assessment of her attempt to bounce others into a leadership challenge by launching her own, a plan scaled back on Monday to instead involve an attempt to make Starmer set a date for his departure.

“Fundamentally unserious. It has made everything worse,” one MP said. Asked if they had ascertained West’s goals or tactics, another replied: “I honestly don’t know. I think most people are giving her a wide berth.”

It has also come as an undoubted surprise to the public, the bulk of whom had not previously come across the former Foreign Office minister, who was born and raised in Australia and moved to the UK in her early 30s.

West does, however, appear to be sincere on her two main points: that she very much believes Starmer needs to go, and that she is not taking a view about who should replace him.

Friends of West say she became increasingly furious as Labour’s dreadful performance in last week’s elections emerged across Friday and Saturday.

A former London council leader before she entered parliament in 2015, West saw a series of people she knew well in local government lose their seats. Labour also lost control of the borough in which her Hornsey and Friern Barnet constituency mainly sits, Haringey, with the Greens becoming the biggest party.

“She is terrified at the idea of Reform just walking into Downing Street, and thinks the party needs a street fighter to combat them,” one friend said. “Keir has many qualities but he’s not that. She wanted to do something immediately – when MPs go back to Westminster it’s easy to forget the lessons from an election.”

And so West announced on Saturday afternoon that she would seek nominations from the 80 other Labour MPs needed to launch a challenge – the threshold is 20% of the parliamentary party – but in the hope that this would smoke out a cabinet minister to act, rather than her being the replacement in No 10.

“I don’t have a candidate. That’s part of the problem,” she said then, adding perhaps hopefully that her ideal was a cabinet reshuffle, with Starmer “given a different role, which he might enjoy”, such as foreign secretary.

West said she had been “inundated with names” of supportive MPs and expected to reach her target, adding that some senior Labour figures had asked her to delay.

Interviewed the next day by BBC One’s Sunday with Laura Kuenssberg show, she was placed next to Bridget Phillipson, the education secretary, with the host pointing out that West had urged cabinet ministers to act.

“Well, there’s nothing stopping Bridget from standing. Why are all the men better than all the women?” West said, as Phillipson gazed fixedly into the middle distance, her face locked in a mirthless smile. Phillipson replied: “I love you dearly, Catherine, but I just disagree on this one.”

There were, it appeared, a couple of flaws in West’s plan. Aside from the lack of cabinet ministers hurling themselves into the fray, a number of people told the MP they were worried by the idea of an accelerated leadership challenge, which might feel like a stitch-up or a coup.

Some of this came from unions, which traditionally do not move at speed. But there was also the issue of Andy Burnham’s supporters, who want a more relaxed timetable, to give the Greater Manchester mayor a chance to get back into parliament.

Such was the worry about West sparking an immediate challenge that some MPs on the left started planning for Ed Miliband, the energy secretary, to be talked into joining the fray, with the belief that Angela Rayner, Starmer’s former deputy, lacks the support.

This was not a worry for West, who told the New Statesman, with some bluntness: “I really like Andy, but he’s not here on the spot, so he can’t really do it.”

But on Monday, West gave way and changed her demands. She announced that instead she would email every Labour MP and ask if they wanted Starmer to set a September deadline to step down. A big enough list of names would put pressure – if no compulsion – on him to do so.

This has perturbed some of the MPs who had supported her push for an instant leadership challenge, who now face the likelihood of Starmer remaining in No 10 for some months.

West’s hope is that the pressure will be such that Starmer agrees to go, and that a replacement can be found by the time parliament resumes from its summer recess in the first week of September.

But for now, the previously little-known MP finds herself in much demand. A friend said: “She had expected that others would come forward very quickly, and that after a couple of media interviews it would all die down. Now she seems to be putting in a lot of hard work for other people.”