Boris Johnson has been accused of “hammering” workers with a “broken tax system” as the end of the universal credit uplift and the hike to national insurance loom.

The Labour leader, Keir Starmer, accused the prime minister of “making a bad situation worse” by in effect cutting the pay of those including supermarket workers and teaching assistants by more than GBP1,000 a year.

Ahead of a vote called by Labour on the end of the extra GBP20 a week universal credit claimants have received during the pandemic, Starmer told Johnson at prime minister’s questions on Wednesday he was about to “take a system already loaded against working people and making it even more unfair”.

Starmer focused on the claim earlier this week by the work and pensions secretary, Therese Coffey, in a TV interview that those who lose the uplift would only have to work an additional two hours to make up the difference.

He asked Johnson if that was correct, but the prime minister initially refused to say if it was, instead criticising Labour for having previously attacked the universal credit system.

When Starmer pressed him again on whether people would have to work less or more than two hours to make up the difference in earnings, Johnson again pivoted to attacking Labour – and labelled the opposition leader “Captain Hindsight” to cheers from some of his backbenchers, but notably not all.

There are at least a dozen Conservative MPs who have voiced concerns about the universal credit uplift being scrapped.

Starmer sought to exploit this divide, pointing out that some of those who would lose out would in fact need to work nine hours extra a week – or one day – “just to get the money back the prime minister has taken away from them” given the new health and social care levy.

However, Johnson batted away the criticism, claiming the government was taking measures to help wages rise and investing in growing people’s skills.

Some people could not work a full extra day every week, Starmer said, and added the reason the prime minister was avoiding admitting that was because “because of his broken tax system”.

He accused the government of “making a bad situation worse” for working people by “hammering them” with the dual cut of benefits and tax rise.

During PMQs and ahead of a cabinet reshuffle expected to begin on Wednesday afternoon, Johnson was accompanied on the frontbench by his chief whip, Mark Spencer, the Commons leader, Jacob Rees-Mogg, the chancellor, Rishi Sunk, the Scottish secretary, Alister Jack, and the party chair, Amanda Milling.

Behind the prime minister, most Tory MPs were sat close together unmasked – though Theresa May was a notable exception.

Labour MPs chanted “up, up, up” as Starmer said food and energy bills, as well as national insurance and council taxes, were due to rise. Johnson retorted that “panto season has come early”.

Johnson continued: “We’re hiring 50,000 more nurses and we’re just putting another GBP36bn into the NHS and social care, on top of the GBP33bn this government invested when we came into office.

“One in 10 of the people of this country is now on an NHS waiting list. They know that the NHS backlog needs to be fixed, they know that this government has a plan, and they know that Labour has got absolutely nothing to say.”