Boris Johnson has been accused by Keir Starmer of “turning on the poorest” as Britain came out of the pandemic by scrapping the GBP20 universal credit uplift, which the Labour leader committed to replacing.

Backing a call by the footballer Marcus Rashford for the government to abandon plans to cut the uplift, which was introduced during the pandemic, Starmer said the cut came “at the worst possible time because prices are going up”.

“Whether that’s fuel or food, or energy prices, and this is going to drive families and children into poverty and for the government to turn on the poorest as we come out of the pandemic is just so wrong,” the Labour leader told BBC Breakfast.

More than 800,000 people risk being pushed into poverty as a result of a cut to universal credit, which was applied to assessments starting on Wednesday and will take effect in a week. About 100,000 renters in England will be in danger of eviction.

The Resolution Foundation, an independent thinktank focused on improving living standards for those on low to middle incomes, has described it as the largest ever overnight cut in benefits, and the government has been warned it will have a severe impact as it coincides with rising energy costs and food prices.

Starmer reiterated that Labour plans to do away with universal credit altogether, but added that if he was prime minister he would keep the GBP20 uplift until the system was overhauled.

“What we would do in the long term is actually replace universal credit, because one of the problems with the system we’ve got at the moment is that it traps people in poverty.”

Pushed on whether that meant the uplift would stay under Labour until the system was replaced, he said: “It would stay. We wouldn’t make the cut. We would then replace it with something better.”

Ministers are understood to be examining a GBP1bn-a-year increase in benefit payments to cushion the impact of the GBP6bn-a-year cut in universal credit. The uplift was introduced to help struggling families through the Covid pandemic.

Rashford, the England and Manchester United footballer who last year forced Boris Johnson into a U-turn on free school meals, last month urged voters to write to their MPs before the GBP20 boost is scrapped on 6 October.

“Instead of removing vital support, we should be focusing on developing a long-term roadmap out of this child hunger pandemic,” he said. “On 6 October, millions lose a lifeline. It’s a move that Child Poverty Action Group says will raise child poverty to one in three.”