A lack of consultation with children and parents is partly to blame for the fall in pupils having school meals in England, inspectors have said.
School meals are less popular since they became healthier
Ofsted found fewer pupils taking meals in about 70% of the 27 schools visited since healthy eating rules came in.
It called on schools to work to eliminate the factors discouraging pupils from having school meals.
New guidelines announced in March 2005 limited the amount of processed meats, deep fried and high fat foods served.
They also required schools to provide more fresh fruit and vegetables.
The changes were prompted by TV chef Jamie Oliver's campaign.
Burgers and sausages from 'meat slurry' and 'mechanically recovered meat'
Sweets including chewing gum, liquorice, mints, fruit pastilles, toffees and marsh mallows
Chocolates and chocolate biscuits
Snacks such as crisps, tortilla chips, salted nuts, onion rings and rice crackers
But successive surveys by organisations including the School Food Trust, the body appointed to oversee the changes, have suggested take-up fell as a result.
Fewer meals were served in 19 of the 27 schools Ofsted visited, with reductions ranging from 9% to 25%.
Ofsted said the reason for the decline was "complex" and included a lack of consultation about the new arrangements with pupils and parents and poor marketing of the new menus.
It added that some pupils, particularly those from low income families, were put off by higher costs.
The quality of dining areas was also an issue, with pupils not wanting to waste their lunch breaks standing in long queues.
But the fall comes despite the finding that the majority of pupils had a good understanding of what constitutes healthy eating because of their food technology and other lessons.
"In too many instances, however, pupils' knowledge had little bearing on the food they chose."
It added: "The reasons why pupils choose not to eat at school need to be taken seriously, especially if the current strategies are to have a positive impact on the most vulnerable."
Ofsted said schools should monitor the take-up, identify factors discouraging pupils from taking lunches and work to eliminate them.
They should also work "closely and sensitively" with families to advise them on how to provide healthy packed lunches.
And pupils should be involved closely in developing school menus and exploring a wider range of food.
Children's Minister Kevin Brennan urged schools and teachers to consider Ofsted's recommendations seriously, adding that the government wanted to tackle obesity and improve children's eating for the long term.
"We are urging schools to make the most of our £477m investment in raising nutritional standards and keeping prices down; greater focus on diet and practical cooking skills in class; and targeting an additional £150m funding at schools with the poorest kitchen and dining provision," he said.
Liberal Democrat education spokesman David Laws said there was no point in having healthy meals if nobody was eating them.
"The government's worthy aspiration for healthier meals has backfired because of inadequate funding and rushing through changes too quickly," he said.