Former Prime Minister Gordon Brown has urged Boris Johnson to provide 1.5 million children with free meals during the half-term and Christmas holidays.
He said Mr Johnson should "personally" ensure poor households get help.
Mr Brown is backing the campaign led by footballer Marcus Rashford, who says pupils should not be "fed by food banks" while off school.
But the government said it was not its job to subsidise children's meals "regularly" outside term time.
An online petition set up this week by Mr Rashford, a Manchester United and England forward, calling for an end to child food poverty, has attracted more than 200,000 signatures.
It says free school meals should be given to all under-16s in England whose parent or guardian receives Universal Credit.
The petition also asks the government to provide meals and activities during "all holidays".
Mr Brown told BBC Breakfast the cost of providing the 1.5 million worst-off children with free meals at half-term and Christmas would be around £20m a week.
This was a "relatively small sum of money", he argued, adding that it would be "unfair" if those who usually relied on free school meals were to go hungry at a time when household budgets are being stretched because of the pandemic.
"This is about social conscience," Mr Brown said. This is about compassion and it's about care."
He also backed Mr Rashford's longer-term proposal to extend free school meals to all children whose households rely on Universal Credit.
There were currently four million children living in poverty, while only 1.5 million received free school meals, the former prime minister told Breakfast, adding: "That doesn't make sense to me."
During lockdown the government provided vouchers to families whose children qualify for free meals.
It had insisted this would not continue outside of term time but changed its mind after Mr Rashford's campaign over the summer.
"Marcus is a great guy," Mr Brown said. "He's speaking from the heart."
A government spokesperson said: "We took the decision to extend free school meals during the pandemic, when schools were partially closed during lockdown. We're in a different position now, with schools back open to all pupils.
"It's not for schools to regularly provide food to pupils during the school holidays. We believe the best way to support families outside of term time is through Universal Credit rather than government subsidising meals."