For months, Boris Johnson’s plan to deliver on a pledge to solve the social care crisis was shrouded in secrecy: names were added one by one to a list held by the cabinet secretary of those brought in to develop parts of the policy so that in the event of a leak inquiry, he knew – in the words of one insider – “who to pin against the wall”.

Even the day before the announcement, many cabinet ministers were still in the dark. The key architects – the prime minister; the chancellor, Rishi Sunak; the health secretary, Sajid Javid – had been frantically negotiating a final settlement over the weekend that was just ambitious enough for their nervous colleagues to swallow.

But when the big reveal was finally made on Tuesday – a hike in national insurance raising £12bn a year and taking the tax burden to its highest ever level – the feared mass rebellion melted away.

Discussions around the cabinet were “wide-ranging” with some dissent expressed, cabinet sources admitted – but in the main, sceptical senior members of the government held their tongue, partly due to the dangled threat of an imminent reshuffle.

But despite the lack of any resignations, the anger among Conservative MPs whose affection their leader craves is growing and the looming danger for Johnson will only be compounded by a series of tricky issues over the coming weeks: introducing vaccine passports, cutting the universal credit £20 uplift and extending emergency powers granted under the Coronavirus Act.

“The PM’s moved one notch further down in quite a lot of people’s estimations,” a frontbencher conceded. “He won’t get unlimited chances. He’s going to try and make me eat shit for a number of things, having just made me eat shit over national insurance contributions (NICs) and I’ll probably let him off – but it won’t be forgotten.”

While Downing Street will have considered the week to have gone better than expected and are privately relaxed about restless MPs (so long as they remain anonymous, limiting the potency of such attacks), No 10 are wary the battle over social care is not yet over.

Labour has just regained a two-percentage-point lead over the Conservatives for the first time since January in a YouGov poll, and there are concerns the £12bn raised through the NICs increase will not eliminate the NHS backlog before the money is meant to begin shifting to social care in 2023. “Red wall” Tories have begun speaking openly about the unfair impact they expect the tax to have on poorer communities. And in the two years before the £86,000 social care costs cap comes into effect from October 2023, many people will lose their home – threatening yet more damaging headlines. As all those effects are felt, and the next general election draws closer, pressure on Johnson will grow.

Johnson had hoped to unveil the social care reforms before the summer recess in July. But that was put on hold when he and Sunak were forced into self-isolation. They had agreed a spending cut was not an option. “Rishi kept up the argument that it was his duty to leave public finances in a strong position for the next generation, so borrowing more would have been irresponsible,” a source with knowledge of the Treasury discussions said. “NICs was agreed pretty early on in the thinking that it was the only viable option to raise the quantity of money needed.”

The idea of badging the tax rise as a new “health and social care levy” was also agreed before the summer. Over the ensuing weeks, negotiations continued over the level at which it should be set, with Javid arguing for more spending, aided by hair-raising predictions that NHS waiting lists could balloon to 13 million.

Johnson’s team believed that while raising taxes – and directly breaking a manifesto promise – may be unpopular with voters, allowing millions of people to go without the treatment they needed would be a political disaster.

At least three cabinet ministers – Jacob Rees-Mogg, Liz Truss and David Frost – are understood to have voiced concerns at the Conservatives being at risk of losing their status as the party of lower taxation. However, the NICs rise wasn’t “enough of a red line” for them, several government sources admitted. One said the cabinet understood it was a personal priority for the prime minister, and that by raising NICs now, there were three years left in the run-up to the next election where Johnson and Sunak may look to cut taxes elsewhere.

In an attempt to shore up support before Wednesday’s vote, the prime minister rang around members of the 2019 Tory intake, among whom he has a huge amount of goodwill because they feel they owe their place in parliament directly to him. But some still rebelled, and more traditional Tories are becoming increasingly disfranchised.

“I’m thinking of joining the TaxPayers’ Alliance,” a senior government source said, in a reference to the lobbying group for lower taxes. Another insider said Johnson needed to “work out what post-pandemic Conservatism means – because if I was comparing this government with Harold Wilson’s [Labour government], there’s not much in it”.

A Downing Street source said Johnson came back to No 10 after announcing the policy and hearing MPs’ reactions on Tuesday and said: “Now we’ve got to get on with this, we’ve got to get it done: we’ve got to make sure that we spend every penny well and people get the treatment they need.”

Some Conservatives have also been buoyed up by what they see as Labour’s disarray over social care, with Keir Starmer criticising the NICs rise but unable to set out his own alternative.

A briefing pack given to Tory MPs defending the levy on the media rounds revealed the lines of questioning and attack they anticipated, however: “Isn’t this just piling costs on to businesses / effectively a jobs tax at a time when businesses are only just emerging from the pandemic? Why are you raising tax on hard-working people to subsidise the wealthy? Why don’t you use income tax which is more progressive and taxes broader sources of income?”

Apart from secrecy over the policy – which some said was the highest since the days of Brexit – another major factor in keeping would-be rebels on side was believed to be the suggestion of a reshuffle.

All-staff briefings in departments were planned, events introducing ministers were well-advanced and office space in No 10 was booked out – all on the presumption Johnson would carve out major changes to his top team on Thursday, which Downing Street did not deny at the start of the week.

It was dubbed the “heist of the century” by an official when the reshuffle failed to materialise. The fear among senior Conservatives is that the same threat will be dangled over their heads for weeks, or even months.

“We won’t keep our powder dry indefinitely,” one warned.