A government amendment to the Children and Families Bill, laid in the House of Lords on Thursday, legislates the provision of free school meals to all children in reception and years 1 and 2.
Last year, the government announced plans for all infant-aged primary school pupils to receive free school meals from this September up until July 2016.
The new legislation removes the scheme's cut-off point, and also includes a new power that will allow schools to extend the offer to older children in the future.
Details of the amendment were disclosed in a joint letter from Education Secretary Michael Gove and schools minister David Laws sent to head teachers.
It reveals that schools will recieve £2.30 for every meal served to a pupil, and that small schools will receive a share of a £22.5m transitional fund to help them prepare for the implementation and delivery of the free school meals.
This is in addition to a £150m fund announced by Deputy Prime Minister Nick Clegg in December to improve schools’ kitchen and dining facilities.
The money was set aside to help schools prepare for the provision of universal free school meals from September – an extension to Clegg’s original initiative, announced last year, where children were only eligible for free school meals if their parents received state-funded benefits.
Matthew Reed, chief executive of The Children’s Society, has welcomed the decision to place a legal duty on schools.
He said: “The announcement that this will be enshrined in law helps protect this benefit for future generations.
“We think this legal duty is a great foundation for the government to build on its achievement and extend free school meals to the 500,000 older children living in poverty who won’t get a free meal.”
Linda Cregan, chief executive of the Children's Food Trust, added: "This is really fantastic news.
"The Children's Food Trust has always believed all children have the right to have a nutritious balanced diet and putting both the funding and legislation in place is a high step towards this.
"Making sure our children have access to free school meals is vital for them to stand the best chance of reaching their full potential and growing up healthy."