Parents without the "time or inclination" to make breakfast for their children are blamed by teachers for more pupils going to school hungry.
A survey from Kellogg's found that four out of five teachers in England have seen examples of pupils starting school without having eaten any breakfast.
"Parental apathy" was identified as the biggest single cause - followed by a shortage of money.
Almost a third of teachers have brought in their own food to feed pupils.
According to the survey, about one in six primary teachers are spending £24.99 per month of their own money on food for their pupils.
Last month, teachers at a primary school in Bristol took over the funding of breakfasts for 130 pupils after a charity providing the meals went bust.
Kellogg's is best known for its cereal brands - but a spokeswoman emphasised that this was not about promoting its own goods, but was a response to its own grassroots work in schools.
The firm's charitable trust has been funding school breakfast clubs, where pupils can get subsidised meals, since 1998.
At present it is supporting more than 500 breakfast clubs across England - but says it is receiving a rising number of requests from other schools worried that pressure on budgets will force the closure of their breakfast clubs.
A report from Kellogg's, accompanying the survey of 500 teachers' experiences, says that without breakfast clubs many more pupils would miss out on a meal in the morning.
More than half of teachers believed that the problem of children arriving hungry at school was getting worse.
They believed that a lack of money is a major problem for some families - but a bigger problem was the failure or inability of parents to provide a breakfast for their children.
"This means that, in many families, parents are leaving children to fend for themselves in the morning. This is because some parents simply don't have the time or inclination to prepare breakfast, let alone supervise their children or encourage them to eat it," says the report.
This problem is worse among primary pupils, says the Kellogg's report, because they find it harder than secondary pupils to "fend for themselves".
The lack of food before school - or only snacking on unhealthy food - means that pupils arrive unable to concentrate and more likely to behave badly, say teachers in the survey.
Gill Harding, head teacher at Primrose Hill Community Primary School, Manchester, says in the report: "Some children at our school don't have their first meal of the day until lunchtime.
"Often this is because their parents aren't aware of what a healthy breakfast is, which is why many children - and their parents - turn up at school eating a large chocolate bar with a can of fizzy pop. In other homes, parents simply don't get up early enough to prepare it."
Karin Woodley, head of the ContinYou education charity, said: "Many families are really struggling financially and, in extreme cases, this means that there simply isn't enough food to go round. Breakfast clubs can provide a lifeline for these families so we're extremely concerned to hear that many are being forced to close."
The issue of poor diet among school children prompted a campaign by celebrity chef Jamie Oliver - and saw the introduction of strict nutritional guidelines.
But Mr Oliver has attacked the decision to allow academies not to comply with these guidelines.
A Department for Education spokeswoman said it was "worrying" to hear of any child going hungry, insisting that the government had protected the schools budget and introduced the pupil premium to support the most disadvantaged pupils.