Children 'bringing cold chips to school' for lunch
Some children are coming to school with cold chips or just a packet of biscuits in their lunchbox, experts say.
An online survey of 250 school, youth and health staff working with children in England suggests many go without enough to eat during the school day.
The Children's Food Trust's poll found 68.1% had seen a rise in the proportion of families struggling to feed their children in the past two years.
Lunchboxes now contain less fruit and more junk food, it suggests.
Of the staff working in schools, 47.5% said they had seen a change in the food in children's lunchboxes as household budgets got tighter.
One staff member said they had seen "poorer quality sandwich fillings, sometimes just margarine".
Another said there were "fewer processed items - more leftovers or store-cupboard items".
But he added: "In some ways it is healthier, but some families only give cold cooked rice or cold chips with fish fingers or similar."
There were also references to more junk food, sweets and chocolate appearing in lunchboxes, and less fruit.
The snapshot survey also found 84.6% of the professionals who chose to take part in the survey had seen children without enough to eat during the course of their work.
Of those who said this, 84.8% said it applied to about a third of the children they worked with.
Children's Food Trust chief executive-designate Linda Cregan said too many people who worked with children were having to go above and beyond the call of duty to try to protect children from the effects of hunger and poor diet.
She added: "Of course it's a parent's responsibility to make sure their child eats well.
"But as this and other surveys have shown, the reality is that this can be an enormous struggle.
"Whether we like it or not, people working in these jobs are at the front line of helping parents on this, so they need the right support.
"As local authorities develop their public health plans, ring-fencing funding to support children's nutrition would be a good starting point.
"This could be used in all sorts of ways - training on cooking skills for local organisations working with families, subsidising good school food, breakfast clubs in schools or grub clubs for the holidays - but making that explicit commitment is vital."
Pupils at Priory School in Lewes told the BBC's School Report project their lunches were generally quite good.
"I haven't seen people with chips in their lunchboxes - but the school does do chips on Fridays though. I guess people just get that," said Flora, aged 14.
Ellen, aged 13, agreed that most people were quite healthy: "I take a piece of fruit to school every day."
School dinners were easier, quicker and nicer because "you can get hot food", according to Ossia, 14.
And 13-year-old Safi said: "Packed lunches are cheaper. I can buy in bulk and have the same thing every day."