Many children would start school hungry without breakfast clubs, teachers have claimed.
A new survey suggests these clubs are the only way many youngsters can get a meal before lessons.
More than half (54%) of the 552 school staff questioned by the Association of Teachers and Lecturers (ATL) said their school or college provides a breakfast club for pupils.
The overwhelming reason for children to attend these clubs is because their parent or carer goes to work early, cited by 76.8% of those questioned. But other factors, such as lack of money, also play a part. More than two-fifths (44.7%) said they believe the main reason pupils attend a breakfast club is because it is the only way the youngster will get a meal in the morning.
Around a fifth (22.6%) said children attended due to lack of money at home because parents or carers are unemployed and 15.2% cited lack of money at home due to changes or cuts to benefits. Around one in six (17.6%) said pupils mainly attend breakfast clubs to socialise.
The survey found that teachers believe that offering breakfast to pupils often helps improve their concentration and ability to learn. One primary school teacher from Kent told the survey: "Although there is a charge for our breakfast club, we have accessed funding for those pupils on free school meals and the breakfast club had an effect on their attendance, concentration and being in school for the start of lessons."
ATL general secretary Dr Mary Bousted said a nutritious meal at the start of the day has a "huge impact" on pupils' ability to learn.
"Many schools do everything they can to ensure children eat well during school term-time. But there are many children living in poverty, who we fear won't be getting a decent meal a day in the holidays and this is something the Government urgently needs to address," she said.
A Department for Education spokesman said: "We know how important it is for children to have a good breakfast. We want schools and local authorities to use their budgets to best meet the needs of their children. Many provide breakfast clubs which offer a free or subsidised meal to children from poorer families.
"The Pupil Premium, which will double to £2.5 billion in 2014-15, targets extra money to help schools provide support such as this to the most disadvantaged children. The Free School Meal scheme also ensures that these children have access to a nutritious lunch every day."