The mayor’s office said it estimated the one-off funding could help more than 270,000 children in the capital during the 2023-24 academic year.
A spokesperson added the plans were also expected to save families about £440 per child over the year.
Charities and unions have welcomed the news but said more action was needed.
The mayor’s office said the scheme, which will be implemented in September and run during term-time only for the length of the academic year, was “one-off funding from additional business rates income”.
A spokesperson said funding for the project was made possible because council tax and business rates returns from the capital’s local authorities were higher than originally forecast in the mayor’s draft budget proposals.
Mr Khan officially announced the plans during a visit to his old school, Fircroft Primary in Tooting, south-west London
‘Game-changing’ plans
Mr Khan, who received free school meals himself, said: “The cost of living crisis means families and children across our city are in desperate need of additional support.
“I have repeatedly urged the government to provide free school meals to help already stretched families, but they have simply failed to act.”
He continued: “The difference they can make to children who are at risk of going hungry – and to families who are struggling to make ends meet – is truly game-changing.”
The launch follows similar decisions by London councils in Newham, Islington, Southwark and Tower Hamlets to offer their own universal primary school free school meals.
Last month, Westminster City Council also began providing free school meals for primary pupils in a scheme set to run for at least 18 months.
Charities and teaching unions have welcomed the plans, but some have urged central government to step up wider support.
‘A huge relief’
Barbara Crowther from the Children’s Food Campaign said: “We applaud the mayor for announcing this vital nutritional safety net for every single primary school child in London for the coming academic year.
“However, healthy school food for all must not just be an emergency measure. It should be a core part of a fully inclusive education system for the long term.”
Victoria Benson, chief executive of the single parent charity Gingerbread, said many parents had told them their children had gone without food because they were struggling with the cost of living.
“It will be a huge relief to many parents that their child will now be fed at school,” she said.
However, youth education charity Impetus said the move may make it harder for schools to find out who was eligible for free school meals.
Currently, an application for free school meals triggers pupil premium funding, which is used to improve education outcomes for disadvantaged children.
“We are concerned that the unintended consequence of this important intervention could end up disadvantaging the children who need it the most,” director Steve Haines said.
‘Government inaction’
Meanwhile Kevin Courtney, joint general secretary at the National Education Union (NEU), said the scheme would be “a much-needed lifeline” that could also help “attainment and educational outcomes”.
“Children who have access to a healthy hot meal every day are better able to focus, connect with their peers and build bright futures”, he said.
“The government must now end its inaction and commit to funding free school meals for all in primary schools across the rest of the country, and long term.”
Chief executive of Child Poverty Action Group, Alison Garnham, described the scheme as “fantastic news” but added it “highlighted the government’s failure to act for children”.
“It really is for ministers now to respond to public support for free school meals and ensure that every child in the UK has a free, balanced meal in the middle of the day,” Ms Garnham said.
A Department for Education spokesman said more than two million schoolchildren had received a free meal since 2010, “thanks to the introduction of universal infant free school meals plus generous protections put in place as benefit recipients move across to universal credit”.
“Over a third of pupils in England now receive free school meals in education settings, compared with 1.1 million in 2009, and we have made a further investment in the national school breakfast programme to extend the programme for another year, backed by up to £30m,” he continued.
The government added the energy bills support scheme had also provided £400 discount to millions of households during winter, with further support for the most vulnerable people.
Co-founder of Leon Restaurants, Henry Dimbleby, hailed the announcement as “absolutely fantastic”.
He told BBC Radio 4 similar schemes had been trialled in 2013 in Newham and Durham, which had “significantly improved academic performance not just for those who weren’t on free school meals before, but actually more even for the children who were already on free school meals because it changes the culture of the school.”
Emma Best, a City Hall Conservative health spokeswoman and London Assembly Member, said: “While I welcome more children having access to free school meals this year, the reality is that many lower income families will be hit hard by a 57% increase in Sadiq Khan’s council tax since 2016 and his £12.50 daily ULEZ charge.”
She said the school meals scheme had “completely missed secondary school pupils”.
“If the mayor genuinely wants to help the poorest families, he should be focusing on those most in need across all schools,” the spokeswoman added.