One in five parents on low income are said to be unaware their children are entitled to free school dinners worth up to £700 a year.
According to the School Food Trust, 22% of parents on a low income questioned were not aware their child was entitled to the support.
On average school meals cost £1.85 in state secondaries and families who are eligible could save £703 a year by signing up and claiming what is rightfully theirs, the trust said.
The survey also found that 39% of parents with an income of less than £16,000 a year said the money saved would put the extra money to good use by paying urgent bills.
Families qualify for free school meals if their household income is less than £16,040 and their parents do not get working families tax credit.
But the research found that eligible parents are still giving lunch money of £3 a day, wasting £1,140 a year.
Trust chief executive Judy Hargadon said: “As the recession bites and unemployment rises, cash strapped families need to be making all the savings they can; we would urge any parent to check with their school or local council and start claiming what’s theirs.”