The Children's Society said: "One million children in poverty who could benefit now won’t".
Currently, all families on Universal Credit can claim free school meals, unlike the rest of the population who are only guaranteed free meals in reception, years one and two.
However, under the new law, children in year three and above on Universal Credit in England will not generally be eligible if their parents earn more than £7,400 a year in net income, excluding benefit money.
The law was passed in a House of Commons vote last week by Conservative and DUP MPs. Shadow education secretary, Angela Rayner, warned the government is "pulling the rug" from under poor families; an accusation dismissed by Conservatives as "scaremongering”.
For children in school and under the care of families alreeady on Universal Credit, free lunches will continue to be available beyond April 1 - no matter how much they earn - until 31 March 2022 or "the end of their phase of education", whichever is later.
However, families who aren't currently on Universal Credit but moving to it in the future, will be affected. The new law states that if you earn less than £7,400 a year, your child(ren) will be eligible for free school meals. If you are under the threshold now but exceed it later, children will remain eligible until the previously mentioned cut-off date.
Exemptions end on 31 March 2022 because that's when Universal Credit will stop being ‘rolled out' across the country, which replaced the old benefit system in 2013. Some areas have switched while others are still waiting.
The GMB trade union said the "cruel school dinner cap" will leave thousands of families with the choice of cutting their hours or their children's meals. The union also warned that there are no plans to raise the cap with inflation, meaning that the problem will get more severe year by year unless the law changes.
The government stresses that, despite criticism of the policy, by 2022 it expects 50,000 more children to have free school meals than under the previous benefits system.
On 13 March, House of Lords peers backed a motion tabled by Labour’s Steve Bassam, which calls on the government to delay implementing changes to its free school meals policy until a full impact assessment is carried out.