Only one per cent of primary schoolchildren's packed lunches meet the nutritional standards set for school meals in England, reveals research published ahead of print in the Journal of Epidemiology and Community Health.
Crisps, sweets, and sugary drinks take precedence over vegetables, fruit, and milk based products, the research shows.
On average, around half of UK schoolchildren eat a packed lunch brought from home, equating to 5.5 billion packed lunches eaten every year.
Prompted by concerns that school lunches were not providing sufficiently healthy food choices, new standards setting out the required healthy food groups for prepared meals came into force for all local authority schools in England in 2006.
These specify that school lunches must contain protein rich and low fat starchy foods, vegetables, fruit and dairy products. Meals cannot include sweets (confectionery), savoury snacks or artificially sweetened drinks.
These were followed in 2008 by further government standards on the energy, fat, salt, vitamin and nutrient content for school meals.
The research team collected information on the packed lunch content for just under 1300 children between the ages of 8 and 9 from 89 primary schools across the UK - 76 in England, four in Scotland, six in Wales and three in Northern Ireland.
All the children took a packed lunch to school on at least one day of the week, and almost nine out of 10 ate a packed lunch every day.
The type and quantity of foods for each child's lunch box were recorded/weighed before and after lunch on one day and compared with the government's school meal standards.
Permitted savoury or sweet foods, vegetables, and permitted drinks (natural juice, milk, pure water) were the least likely to be provided. Sandwiches, sweets, savoury snacks and artificially sweetened drinks were the most common items.
Around one in three children were given a sandwich with a low protein filling, and only one in 10 children had sandwiches containing vegetables. A further one in 10 were given a separate portion of vegetables.
The foodstuff least likely to be eaten when provided was fruit; the foodstuff most likely to be eaten was confectionery.
More than one in four children (27%) had a packed lunch containing sweets, savoury snacks, and sugary drinks; a further four out of 10 had sweets and snacks, but no sugary drink. Fewer than one in 10 (8.1%) had none of these foodstuffs in their lunchbox.
Only 1.1% of the children's packed lunches met all the required nutritional standards for school meals; a further 5.1% met the 2006 five healthy food group standards.
Fewer than half of children's packed lunches met the government's 2008 nutrient standards, including levels of vitamin A, folate, iron and zinc.
On average, girls tended to be given, and eat more, healthy foodstuffs than boys, and children at schools with fewer pupils eligible for free school meals had healthier packed lunches.
The authors say their findings provide "evidence that the quality of food in children's packed lunches is poor... Few lunches contained all five healthy food groups, but most lunches contained restricted foods and drinks such as crisps and cakes."
The new standards for school meals are producing "drastic improvements" in lunches provided by primary schools, but the same cannot be said for packed lunches, they add.