Children will have to pay up 17% more for their school dinners this year compared to last, a survey has found.
Consumer watchdog Which? also said the quality of school meals needs to improve to encourage more children to eat them and keep costs down after finding that the price is rising in two-thirds of schools across the country in the coming term.
This is leading to concern that it could start to undo the progress made in recent years towards improving children's access to healthier meals.
The research found that parents would rather give their children packed lunches as they believe them to be cheaper, and because their children do not like the food on offer at schools.
Meal prices have risen on average by around 2.5% on last year but some local authorities have increased prices by far more.
School dinners managed by Poole Borough Council are the most expensive in the country at an average of £2.50 this September.
Doncaster Metropolitan Borough Council has increased prices by 17% to between £1.70 and £2 a meal while Lewisham Borough Council has risen by 14%, so that school dinners will cost from £1.40 to £1.60.
The local authority with the biggest increase was Bolton Metropolitan Borough Council at 25%, although its prices still remain the lowest in the country at £1.25.
It is estimated that in order to keep costs down, 55% of students would need to take school meals. However, the research found that just 45% of school pupils in England currently have them.
In some areas, including Wokingham District Council and West Sussex County Council, it is as low as 25%.
Richard Lloyd, executive director of Which?, said: "At a time when many people tell Which? their number one concern is rising food prices, it will come as an unwelcome surprise to hard-pressed families to see that some local authorities are increasing their prices by as much as 17% - well above inflation.
"School meals in most areas are still a relatively low cost and low hassle way to provide a decent lunch for your children.
"But if schools cannot find ways to protect the extra funding that has gone to school meals and increase the number of children taking them up, there's a real risk of even more price hikes or a drop in standards, undoing the progress that has been made over the past five years."