Less than 2% of primary school packed lunches in England meet nutritional standards, according to new research.
Researchers from the University of Leeds found that lunchboxes are full of high levels of sugars, salts, and saturated fats - none of which are healthy.
They found that just one in five boxes contained a vegetable or any salad, and just 1.6% meet current nutritional standards.
Over half - between 52% and 60% - of the lunchboxes examined contain too many snacks, while nearly half (46%) included sugary drinks.
The research, commissioned by Flora, found that the quality of children's lunchboxes had barely improved in the last decade.
Flora is working with the chair of the All Party Parliamentary Group of School Food (APPG), Sharon Hodgson, to improve awareness around healthy eating for children.
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Campaigners want to reduce the amount of sugary drinks, crisps and chocolates being brought into school, and are urging the government to put packed lunches at the heart of its focus on healthy school food.
Mrs Hodgson said: "The research highlights the need for more action to be taken on food put in children’s packed lunches, something which the school food APPG has recently called for.
"Despite positive moves with regards to the food provided as part of a school meal, food brought in by children in their packed lunches is lagging behind. Therefore we need more action to be taken if we want to see positive changes."
Jo Nicholas, head of research at the Children's Food Trust, said: "We've got to do more to help parents and schools with this.
"Putting a packed lunch policy in place can be tough, but every school allowing packed lunches needs one if we're going to make life easier for parents and give kids a consistent message."
What do you give your children to take to school in their packed lunch?
ITV News asked parents on Facebook what they give their children for packed lunch.
Parents have expressed concern about the quality of food available at schools
Parents have been united on the Facebook page with outrage at schools' strict rules on lunchboxes, when the school dinners include unhealthy options for their children.
Chelsea Dixie Forwood said: "The school my son was in had strict rules on lunch box items - no crisps or chocolate. Yet if you have a school dinner, each child gets a dessert like cake or ice cream."
Sarah-Jane Colley added: "When my daughter who has school dinners comes home, she says she's had pizza and chips followed by chocolate fudge cake and custard."