The Conservatives have set aside just under 7p per pupil for its manifesto pledge to give all primary school pupils free breakfasts, in what food experts have labelled a “black hole” in the government’s manifesto calculations.

The party’s manifesto, launched last Thursday, scraps universal infant free school meals (UIFSM), which cost an estimated £600 million a year, in favour of free breakfasts for all primary pupils, which a press release today said will cost just £60 million a year.

But critics have calculated that if the country’s 4.62 million primary state school pupils were fed a free breakfast on this budget for 190 school days each year, each meal would have to cost no more than 6.8p.

Even if just half of those pupils took up the offer of free breakfast, these meals would cost just 13.6p each.

Aisling Kirwan, the founding director of the Grub Club, a school-based social enterprise that provides cooking lessons for pupils in poorer areas, said that a nutritious meal costs 25p per pupil on average – which even then would only amount to porridge with milk.

A more filling portion, which would include bacon, two sausages, one egg and bread, would cost 85p per portion.

“Clearly there’s a huge disparity between the realistic costing and that put forward by the Tories,” she said.

Half of low-income pupils go hungry at breakfast, and a further fifth eat breakfasts with little or no nutritional value. Poorer pupils must be able to get hold of the right nutrients at breakfast clubs, Kirwan insisted.

Dr Rebecca Allen, director of think tank Education Datalab, said schools were looking at a bill in the region of £400 million once costs of paying a teaching assistant to oversee the breakfast club were included.

The government had not just “slightly” underestimated the free breakfast policy costs, but underestimated them by between five- and 10-fold, said Allen. The cost would be closer to estimations if breakfast clubs were held in morning lessons rather than before school began, she added.

The Conservatives first proposed the end of UIFSMs after the Institute of Fiscal Studies and the Education Endowment Foundation conducted research to show that free breakfasts for primary pupils were more effective for learning outcomes, and less expensive, than free lunches for pupils in their first three years of school.

Ellen Greaves, a senior research economist at IFS who evaluated a trial on breakfast clubs by the EEF, said it was “really clear” that breakfast clubs “had a positive impact at a relatively low cost”.

“It’s a lower cost than universal free school meals for infants, which is a national policy.”

Labour came under fire for its manifesto commitment to spend £1.5 billion extending universal free lunches to all primary pupils after councils failed to back Jeremy Corbyn’s claim it had been proven to boost attainment and tackle obesity.

But now the Conservatives appear to have made their own miscalculation. According to Schools Week’s sums, if each pupil were given breakfast at 50p per portion, it would cost £723.9 million per year to fund – a cool £663 million more than currently budgeted for.

According to the government’s own Healthy Eating Regulations for schools, a nutritious breakfast consists of milk-based drinks or yoghurt, fruit and vegetables, bread and non-flavoured or coated cereals.