If Keir Starmer wasn’t already aware of the strength of feeling among Labour MPs at the government’s plan to slash the welfare bill, it was brought home to him on Thursday evening.

Party whip Vicky Foxcroft – someone whose job it was to persuade Labour MPs to back the government – resigned in protest, saying she could not vote for reforms which will hit the incomes of disabled people.

In her letter to the PM, she said: “I have wrestled with whether I should resign or remain in the government and fight for change from within. Sadly it now seems that we are not going to get the changes I desperately wanted to see.

“I therefore tender my resignation as I know I will not be able to do the job that is required of me and whip – or indeed vote – for reforms which include cuts to disabled people’s finances.”

As a former shadow minister for disabled people, and a party loyalist, Foxcroft’s resignation will have sent a shiver up spines in No.10.

The government’s plans, set out in a bill published last week, will make it harder for people to claim Personal Independence Payment (PIP) and reduce the health element of Universal Credit.

Ministers insist it will make the benefits system more sustainable by cutting costs by £5 billion, while also encouraging more people into work.

But there is widespread anger among Labour MPs, more than 100 of whom have written to the government’s chief whip, Alan Campbell, telling him they cannot support the measures.

In a bid to buy off some rebels, work and pensions secretary Liz Kendall announced that those losing their PIP will continue to receive their money for 13 weeks. For many MPs, however, this is nowhere near enough to win their support when the Commons vote takes place on July 1.

Between now and then, the government’s remaining whips will engage in a concerted campaign of arm-twisting in an attempt to reduce what at the moment is on course to be the biggest Labour rebellion of Starmer’s time as PM.

One rebel MP told HuffPost UK that more than 100 of their colleagues will abstain, with a further 50 potentially voting against.

“It might not be enough to kill the welfare cuts, but it’s definitely enough to kill Keir’s authority over his MPs,” the backbencher said.

The tactics allegedly being employed by the whips as they try to persuade Labour MPs are also proving to be controversial.

Some potential rebels claim to have been threatened with losing the Labour whip, the same fate which befell seven MPs who voted against the King’s Speech last July.

Others have been told their chances of promotion to the ministerial ranks will be zero should they vote against the government, while others have been warned that they’ll lose campaign funding come the next election.

“I’ve huge respect for the whipping process, but what they’re doing is not whipping, it’s Labour students stuff,” said one MP. “They’re trying to divide and conquer.”

Claire Reynolds, No.10′s political director, recently met with a group of Labour MPs who were elected last year and warned them against walking through the same voting lobbies as Nigel Farage.

“It’s pure tribalism,” said one MP. “You can’t run the Parliamentary Labour Party like it’s a student union. They’re trying the carrot and stick approach, but the carrots aren’t working and their sticks keep breaking.”

Luke Sullivan, who was a special adviser in the Labour whips’ office between 2008 and 2021 and is now a director at Headland Consultancy, said he doubted whether his former colleagues will really be using such heavy-handed tactics.

“No government whip would ever make any of those threats,” he told HuffPost UK. “Ultimately, every whipping operation I’ve ever been part of has always taken the view that however big the vote is, there’s always a bigger one coming down the track, so you need to maintain relationships with people.”

Sullivan said the chief whip will have a huge spreadsheet setting out where each Labour MP is on the vote, which will dictate the tactics the party employs over the next 10 days.

“At the moment, they’ve got a very good idea where people are,” he said. “As we get closer to the vote, MPs will start coming into No10 and speaking to senior staff about their concerns.

“Others will speak to the ministerial team in the Department of Work and Pensioners. Right up until and including the day of the vote, the whips will arrange for MPs to speak to whoever might be most persuasive for them, all the way up to the prime minister.

“They will be trying to sell the welfare reforms by telling MPs it about getting people back into work and giving them the support they need. It’s going to be a long two weeks.”

Former Tory MP Steve Baker is no stranger to rebelling against his own government, having regularly done so over Brexit and Covid lockdowns.

He also whipped his fellow rebels into line, so knows a thing or two about how to persuade MPs to vote in a certain way.

“There are two categories of MP – those who have firmly made up their mind and will not change it, and those who will change their mind under pressure,” he said.

“The whips will first seek to ensure the MP has understood the policy, understood the trade offs surrounding the policy and understood why the alternatives are all worse.

“They also seek to ensure the member of parliament has considered the consequences for the country, for their constituency, for their party and for themselves if they vote against the government. These are the rich seams which constitute a whipping operation.”

Baker also had some advice for the Labour whips struggling to get their MPs into line.

“Labour cannot allow their members to get a taste for it,” he said.

“If you rebel, the sky doesn’t fall in. All that happens is your colleagues are grumpy with you for a few days and then everyone moves on. Once they realise that, they will keep rebelling. This is the true horror the Labour whips face.

“They cannot once have a large rebellion, because if they do they will have many more.”

Whether they have to employ the dark arts or not, a lot rests on how effective the Labour whipping operation is in the run-up to July 1.

A large rebellion may not be enough to stop the welfare cuts, but could be a portent of even greater danger ahead for the prime minister.