Sadiq Khan has unveiled an emergency scheme to extend free school meals to every primary school pupil in the capital for one year.
The Mayor of London said the one-off £130m programme, which comes into effect from September, aims to help struggling households during the cost of living crisis.
Funded by additional business rates income, it is estimated the move will help around 270,000 primary school pupils and save families around £440 per child.
Currently, households in England receiving Universal Credit must earn below £7,400 a year before benefits and after tax to qualify for free school meals.
According to the Food Foundation, an estimated 800,000 children in England are living in poverty but do not qualify.
Mr Khan told Sky News: "We're hearing stories about parents skipping meals so that kids can eat, teachers bringing in food so their students can eat. And even heart-breaking stories about children without a packed lunch, not on a free school meal, pretending to eat around the lunch table at school to avoid embarrassment.
"So what I'm able to do is announce £130m so every child at a primary school in London from the next school year for at least a year will have a free school meal, a nutritious hot meal during the course of the day."
Mr Khan said that it was a personal issue as free school meals were a "lifeline" for him growing up.
He added: "I remember the feeling of shame and embarrassment because I had to get my free school meals voucher separate from my friends and eat separately from the majority of pupils because we got our voucher late.
"By making it universal, there will be no shame, no embarrassment, no stigma."
Khan puts himself at odds with Starmer
Asked about extending the free schools meals to secondary school children, Mr Khan said he hopes the government sees that "if London can afford to do this with the limited powers and resources we have" then they will "wake up to the importance of all of our children having at least one nutritious meal during the course of the day".
He pointed to research from accounting firm PwC showing that investment in universal free school meals would yield a net economic benefit to society of £2.45bn over 20 years.
Anna Taylor, chief executive of the Food Foundation, called on the government to follow Mr Khan's lead and invest in free school meal expansion in the upcoming spring budget.
She said Mr Khan's announcement was a "monumental step forward for safeguarding children's diets and wellbeing" but that "outside of London, hundreds of thousands of children living in poverty still don't qualify for a free school meal".
Asked about the policy in a briefing with journalists, a Downing Street spokesperson said that the life-long benefits of free school meals are "under review" but "our overarching belief is it should be targeted to those that need it".
Labour leader Sir Keir Starmer also would not be drawn on whether he would expand free school meals to all primary school children - something his predecessor Jeremy Corbyn had promised.
Asked if a Labour government would make universal free school meals a national policy he said: "Well, look, the mayor has put in place his scheme for free school meals. We'll have to look at that.
"There are real issues about hunger at schools. We will have plans. But I'm not going to lay out a manifesto this far in advance of an election."