The letter details how the scheme, which was introduced last year, has helped tackle child hunger and boosts attainment.
It also notes how without UIFSM, families who earn more than £16,190 would have to pay about £400 per child a year for school meals.
In the 2015 Conservative manifesto the party detailed its commitment to the continuation of the scheme.
However, doubt has been cast over the future of UIFSM as the Department for Education said it would be considered as part of the autumn spending review despite the Prime Minister saying he is “proud” of the policy.
Amongst the signatures is Alison Garnham, Chief Executive, Child Poverty Action Group; James Cashmore, Director, Food for Life/Soil Association; and Carmel McConnell, Chief Executive, Magic Breakfast.