Scrapping free school lunches in favour of free breakfasts could have a significant nutritional cost, Nick Clegg has warned, saying Theresa May should take “her inspiration from Jamie Oliver not Oliver Twist”.

The Liberal Democrats will launch a new poster campaign on Saturday to protest against the scrapping of universal free school lunches for primary school pupils under seven in the Conservative manifesto, one of the policies the party authored during their time in the coalition.

The picture shows a child with an empty bowl, with the quote: “Please Theresa, may I have some more?”

Clegg said scrapping the lunches, which will be replaced by free breakfasts, could mean thousands of pupils no longer receive a single portion of fruit or vegetables on a daily basis.

No nutritional guidelines exist for school breakfasts, though schools are expected to provide two to three of the recommended five portions of fruit and vegetables a day in school lunches.

Nick Clegg called the Conservative’s pledge ‘particularly short-sighted’. Photograph: Bloomberg/Getty Images

Clegg said squeezed budgets and current nutritional standards could mean breakfast consists mainly of processed breakfast cereal and toast with sweetened spreads, with no limit on the total amount of sugar children can consume. NHS Digital figures from November 2016 show more than a fifth of reception children were overweight or obese.

“This is particularly short-sighted when we are struggling with soaring levels of childhood obesity,” Clegg said.

“Conservative candidates must make clear whether they will vote to take away lunches from hard-pressed families or stand up to Theresa May for the sake of the education and good health of our children. Theresa May should take her inspiration from Jamie Oliver not Oliver Twist.”

Both Labour and the Lib Dems have committed to extending free school lunches in their manifestos to all children in primary education. Labour has said it will fund its policy by making VAT payable on private school fees.

Pupils eating free school lunches are expected to be given a minimum of five portions of fruit, five portions of veg, five portions of protein – which can include meat, fish, eggs or beans as well as five portions of starchy food, at least one being wholegrain, and five portions of milk or dairy, according to government guidelines.

Rules are less stringent for breakfast provision, advising only against providing food too high in salt, sugar and fat. Schools are also expected to make fruit “available” for pupils but no minimum portions are set.

Packed lunches are also far less likely to meet nutritional standards, according to a 2010 study commissioned by the Food Standards Agency, which found that just 1% of packed lunches met the nutritional standards set for school lunches.