Getty ImagesFree breakfast clubs are to launch at 750 schools across England next week but teachers have voiced concerns that government funding for the scheme will not meet the cost.
From Tuesday, thousands of parents can access half an hour of free morning childcare as part of the trial that will run to July, ahead of an expected national rollout.
But teaching unions have raised concerns the funding is too low.
Education Secretary Bridget Phillipson said the move would help with "breaking down barriers to opportunity".
Labour campaigned on a promise of free breakfast clubs in every English primary school and later tripled funding to £30m.
The scheme will give parents of primary-aged children up to 95 additional hours and save them £450 per year in childcare costs, ministers say.
The headteachers union welcomed the expansion of breakfast clubs, which some schools already run, but said schools have suggested the funding is not enough.
Paul Whiteman, general secretary of the National Association of Head Teachers, said: "While we welcome the intentions behind the programme, the initial feedback we are hearing from many school leaders participating in the pilot is that the funding just isn't sufficient.
"At a time when school budgets are already stretched, most can ill-afford to subsidise this shortfall."
He added it was "absolutely crucial" the funding is addressed before the scheme is rolled out nationally and it has received assurance from the government the funding would be looked at carefully during the trial.
Teaching union NASUWT said the scheme would make a "significant contribution" to tackling child hunger but it needed to be closely monitored to ensure funding issues can be addressed without it impacting other provisions within schools.
Announcing the first 750 schools to join the pilot scheme, the Department for Education said breakfast clubs had "an important role to play in the government's commitment to remove the stain of child poverty".
Education Secretary Phillipson said: "Free breakfast clubs are at the heart of our Plan for Change, making working parents' lives easier and more affordable, while breaking down barriers to opportunity for every child."
Devon tops the list with 25 schools in the programme, followed by England's largest local authority, Birmingham, with 24.
Both Wales and Scotland have programmes to provide free breakfasts to children in primary schools.
Guidance sent to schools taking part in the pilot scheme says they will receive a set-up payment to cover equipment and material.
Under the scheme, schools will then be reimbursed by the government based on attendance at the clubs - a school with 50% participation in the pilot scheme could get £23,000 a year, the government said.
