Free meals should be given to children from the poorest families attending summer holiday clubs, say researchers.
Children are being urged to eat health food all year round
More than 800,000 children who usually get free school meals face six weeks without support, says the Institute for Public Policy Research (IPPR).
Children from low-income families should benefit from nutritious food throughout the year, says the IPPR.
Head teachers welcome the idea but think the government would struggle to justify the £40m bill.
There are about 850,000 children in England who receive free school meals - and when these children attend summer school clubs they should continue to be offered free meals, says the IPPR think tank, to ensure they eat healthy food all year round.
IPPR North director Sue Sterling said: "Extending free meals to holiday clubs for the poorest children would help encourage good eating habits all year round and improve concentration and learning during term time.
"In the holidays, many poor children are eating cheaper and energy dense food but are being nutritionally deprived.
"We shouldn't be blaming poorer parents: we should be helping them meet the cost of healthy alternatives."
Each local council is required by law to provide a free two-course meal for eligible pupils during term time, but there is no obligation during holidays.
A free meal each day over the six week summer break would cost £46.20 per primary school child and £48.60 per secondary pupil, according to the IPPR.
This would cost the government around £40m a year.
Such a move would be beneficial to the youngsters concerned, but could present a funding dilemma for the government, says the Association of School and College Leaders (ASCL).
Martin Ward, ASCL deputy general secretary, said: "If the funding is there, then I'm sure the holiday clubs would be able to provide a healthy meal in the middle of the day by some means.
"We welcome that notion. £40m isn't a large amount, but as with all of these things it's a matter of priorities."
Schools minister Kevin Brennan said the government had invested almost £500m in healthy school meals and was providing free fruit and vegetables to pupils.
"Young people can access out of school hours activities, such as breakfast clubs, both in term time and during the school holidays through the extended schools programme.
"We have recently announced a £265m subsidy scheme to help disadvantaged children access extended schools' services," he added.