All pupils at infant schools in England are to get free school lunches from next September, Lib Dem leader and Deputy PM Nick Clegg has announced.
The change - for children in reception, year one and year two - will save parents about £400 a year per child.
Targeting infants would ensure "every child gets the chance in life they deserve", teach healthy eating habits and boost attainment, Mr Clegg said.
But Labour said the Lib Dems could not be trusted to deliver.
Money is being provided for Wales, Scotland and Northern Ireland to emulate the English scheme, but as education is a devolved issue, it is up to those running schools there to decide whether to spend the money on free lunches.
A Scottish government spokesman said: "We are committed to expanding this provision further and, once we see the financial implications of this announcement for Scotland, we will examine how best to deliver that expansion."
'Feeling the squeeze'
Free primary school meals for all pupils was one of the recommendations of a recent review of school food by two founders of the Leon restaurant chain for the Department for Education.
It concluded that packed lunches were nearly always less nutritious than a cooked meal, and that giving all children free lunches would raise academic standards.
The new policy does not ban packed lunches, but the aim is that having the hot, free option, will boost the numbers of pupils having school dinners.
Mr Clegg said: "My ambition is that every primary school pupil should be able to sit down to a hot, healthy lunch with their classmates every day.
"Millions of parents across the country are feeling the squeeze... I am determined to do all we can to help put money back in the pockets of these families.
"We will start with infant school pupils because teaching healthy habits young, and boosting attainment early, will bring the biggest benefits.
"Universal free school meals will help give every child the chance in life that they deserve, building a stronger economy and fairer society."
The move was welcomed by the National Union of Teachers, who called for it to be extended to all primary school pupils.
Children "do not stop being hungry at seven years-of-age", said general secretary Christine Blower.
Dr Hilary Emery from the National Children's Bureau said: "It's encouraging that politicians have recognised the clear link between a good diet, children's health and performance in education - which is of particular importance to low income families."
At the moment free school meals are available to all children whose parents are on benefits or earn less than £16,190 a year.
Providing them for all infants will cost an estimated £600m and comes after the previously universal child benefit was cut for those earning more than £50,000 a year.
The Lib Dems also announced that poorer college students will be entitled to free school meals - on the same basis as those studying at school sixth forms.
"The news will no doubt be welcomed by disadvantaged students and their parents at a time when family budgets are being stretched to the limit," said Martin Doel, chief executive of the Association of Colleges.
But Labour's Stephen Twigg said: "After three years of broken promises and empty words, people have come to judge the Lib Dems on what they do, not what they say.
"They talk about helping families but they will have taken up to £7bn a year of support away from children by 2015. They talk about helping with school meals after supporting the Tories in scrapping Labour's plans to extend free meals for school kids. You can't trust a word the Lib Dems say."
Marriage tax break
Asked if it was fair to provide free schools lunches for the children of all, irrespective of wealth, Mr Clegg said: "We believe that where we can find the money, even in these difficult times, we need to really invest that money in giving all children regardless of their family background the very best possible start in life."
He said the details of where the money to fund the lunches was coming from would be given in Chancellor George Osborne's autumn statement.
At a briefing ahead of the announcement the Lib Dems suggested they had got the funding for school lunches in return for allowing Conservative plans for a marriage tax break.
The Department for Education ordered a review by restaurateurs Henry Dimbleby and John Vincent into the state of school meals in 2012 following strong criticism from TV chef Jamie Oliver, who earlier led a successful campaign to ban junk and processed food from school canteens.
Mr Oliver's campaign resulted in tight nutritional guidelines and healthy eating policies in many schools for those bringing packed lunches.
But in 2011 he claimed that standards were being eroded because academies and free schools were exempt from national nutritional guidelines.
Mr Dimbleby said he was "absolutely buzzing" following Mr Clegg's announcement.
He said: "Even those who have free school meals already benefit from this change of culture... Hopefully it will be the first step on the road to free school meals for everyone."