Schools could be given legal powers to vet the food and drink in their pupils' lunchboxes under new proposals, TES Cymru has learnt.
The healthy eating in schools measure, which is making its way through the Assembly, will put a duty on government, councils and schools to promote good nutrition.
But a government amendment could go further, giving schools legal responsibility over packed lunches as well as food served on site. Pupils could be banned from bringing fizzy drinks and food containing high levels of fat or salt.
An Assembly government spokesman said: "We believe governing bodies should retain the flexibility to deal with this issue at their discretion . in line with our general approach to school governance."
Parents and schools in Wales have already been issued with a booklet offering them advice on how to pack healthier lunchboxes.
The measure is the first to be proposed by a backbench member since the Assembly was given primary legislative powers in 2006.
Jenny Randerson, the Liberal Democrat education spokeswoman, won a ballot of Assembly members to take her measure forward in 2007. She said it would strengthen, and give a legal framework to, the government's Appetite for Life plan, which sets out nutrition standards for school meals.
Jane Hutt, the education minister, said the measure's objectives were "entirely in line" with those of the government.
But Ms Randerson fears the measure could be weakened after other amendments were rejected in favour of government amendments during a cross-party committee meeting last week.
The rejected amendments included placing duties on governing bodies to consult school councils about their healthy eating and drinking plans and to produce regular reports on their success.
Ms Randerson was frustrated that members rejected an amendment forcing the Assembly government to promote healthy eating and drinking within the curriculum.
She warned: "If we don't do this, we are striking at the heart of promoting healthy eating."
But Jane Hutt said powers already exist to ensure the curriculum is geared towards healthy eating.
Speaking after the meeting, Ms Randerson said: "I find it very frustrating that a series of amendments suggested by the original committee, which would have made the measure stronger, have been voted down." She said she would raise some amendments again when the measure is debated in plenary next month.
Meanwhile, primary pupils will not be given free fruit and vegetables after a European Parliament scheme was deemed too expensive.
The Assembly government said it could not afford the match funding required, but did not rule out being involved in future. Around 1,000 schools in Wales already run fruit tuck shops.