A UK government minister has said he is “not comfortable with breaking any manifesto promises” as the prime minister prepares to announce an increase in national insurance contributions to fund health and social care and limit a rise in the state pension.

Nadhim Zahawi, the vaccine deployment minister, was defending plans to fund an overhaul of social care and tackle the NHS backlog, which have attracted criticism from Conservative frontbenchers, former chancellors and the party’s so-called “red wall” MPs.

It is widely expected Boris Johnson will later on Tuesday reveal plans to break two manifesto commitments: an increase of national insurance by 1.25% and to break the pensions triple lock – a pledge to increase the state pension each year in line with the rising cost of living, increasing average wages, or 2.5%, whichever is highest.

Johnson is facing a mounting rebellion from his MPs over the proposals, with one Tory frontbencher telling the Guardian they were considering their position, questioning the point of serving a government that was not pursuing the 2019 party manifesto.

Speaking on Sky News, Zahawi said he was “not comfortable with breaking any manifesto promises” but refused to be drawn on the specifics of the funding.

He also conceded that the NHS backlog would “increase before it gets better” despite an extra GBP5.4bn cash injection announced overnight for the NHS for the next six months.

Asked whether the reforms would work, Zahawi said: “You have to at least have a really good go at making sure you fix the system.

“It would be presumptuous, and I think completely arrogant, to say ‘of course it will fix the problem’. The right thing to do is deliver the reform and the investment into social care – you’ve got to make sure that is operational.

“But I’m being respectful and cautious and not being arrogant to say ‘of course, yes, everything will be fixed in five minutes’ – it won’t, in terms of the NHS backlog.

“We will tackle it, we want to reduce it but it will increase before it gets better, but we are putting the resources, the money that we announced yesterday – the additional GBP5.4bn, taking the total for this year alone of additional support for the NHS to GBP34bn – will go some way to addressing the backlog.”

On Tuesday, the prime minister will frame the tax increase, which could raise up to GBP10bn a year with a 1.25% rise in national insurance contributions (NICs) for employers and employees, as essential to combat the NHS waiting list crisis.

In the long term, funding will be used for social care costs once a patient reaches a costs cap, thought to be about GBP80,000. Under the current system, anyone with assets of over GBP23,350 funds their care in full, and about one in seven people pay over GBP100,000.

After significant cabinet wrangling, plans were signed off on Monday night for the new social care and NHS funding package that Johnson will present to cabinet on Tuesday followed by a statement to parliament.

Sir Andrew Dilnot, who led a landmark commission into the provision of social care under David Cameron, and reported his findings in 2011, said the proposed reforms had the potential to be “transformational”.

He told BBC Radio 4’s Today programme: “I think there’s a real chance we’ve finally got there. This is something that has been necessary, reform here has been necessary for at least 25 years and so if we have got there then that would certainly be a day to celebrate.”

Asked if he was comfortable breaking manifesto pledges, Zahawi said he did not want to pre-empt the prime minister. “I’m not comfortable with breaking any manifesto promises,” he added.

Ahead of the announcement, the prime minister will warn MPs that the NHS is at crisis point. “The NHS is the pride of our United Kingdom, but it has been put under enormous strain by the pandemic. We cannot expect it to recover alone,” Johnson said in comments released overnight.