Any child in England whose parents receive Universal Credit will be able to claim free school meals from September 2026, the government has said.
Parents on the credit will be eligible regardless of their income. Currently, their household must earn less than £7,400 a year to qualify.
The government says the change will make 500,000 more pupils eligible, which the prime minister said would "help families who need it most".
Sir Keir Starmer is facing more calls to tackle child poverty by scrapping the two-child benefit cap, but told the BBC that focusing on free school meals would help boost pupil attainment.
The government said the changes to free school meals would save parents £500 a year and "lift 100,000 children out of poverty".
Sir Keir said the measure was a "down payment" on a broader child poverty strategy, including measures that have already been announced, such as free school breakfast clubs, and those due to be published in this autumn.
Asked by the BBC why he would not first scrap the two-child benefit cap – which prevents most families from claiming means-tested benefits for any third or additional children born after April 2017 – he said "it's really important that we focus on attainment and on the cost of living".
"I'm the first to accept there's no silver bullet, there's no one thing that if you do that there are no other issues," he said. "That's why the [Child Poverty Taskforce] is looking at a broad range of measures."
Education Bridget Phillipson also stressed the plan would lead to higher educational outcomes because "if you're hungry, it's really hard to concentrate".
The Department for Education (DfE) has set aside £1bn to fund the change up to 2029.
In an interview with BBC Radio 4's Today programme, Ms Phillipson declined to say where that money would come from, but said schools would not have to find money for it and that details would be set out in next week's spending review.
She also confirmed to BBC Breakfast that the government was considering scrapping the two-child benefit cap.
The government has also pledged £13m to a dozen food charities across England to "fight food poverty" and said there will be a review of standards so that school meals are healthy.
