Academics from King's College London analysed the food and drink on offer at lunchtime in a nationally representative sample of 146 English primaries over five consecutive days last summer.
They found that only 23 per cent of the schools met compulsory standards, introduced in 2001, at the start of the lunch period. Ten minutes before lunches stopped being served the figure had dropped to just 17 per cent.
From September, tougher nutritional standards are being introduced in response to concerns about school meals raised by TV chef Jamie Oliver.
The report's results were published as a review of academic research commissioned by the Department for Education and Skills. This concluded that nutritional deficiencies affected children's brain performance and behaviour.
The King's College research found that more than half of primaries served starchy foods cooked in fat or oil, such as chips, more than three times a week, against government regulations.
A third failed to offer fruit desserts at least twice a week, as required.
The researchers also examined 7,058 lunches. "They chose too many foods containing fat, and foods and drinks containing sugar, and too few fruits and vegetables and milk and dairy products," they said.
An analysis of what food was actually eaten by the children found that only 57 per cent consumed the requirement for energy set out in the guidelines, 55 per cent for vitamin C, 39 per cent for vitamin A, 37 per cent for calcium, 32 per cent for folate, 30 per cent for added sugars and 15 per cent for iron.
Kevin McKay, chairman of the Local Authority Caterers Association, said schools stood a good chance of meeting the new standards, because they would be better policed by Ofsted, pupils were being educated about healthy eating, and there was more, though still inadequate, funding.
A DfES spokesman said the survey demonstrated the need for the new standards that were being introduced with pound;220 million of extra funding. The package meant there would be no excuse for schools failing to meet them.