The move has reduced teachers to technicians and goes against the findings of a Department for Education and Skills commissioned review which raised doubts about the exclusive use of phonics for children of that age, said Wendy Scott, on behalf of the Early Years Curriculum Group.
Since September schools have been required by law to use synthetic phonics to teach reading. That children should be using phonics by the age of five was a key recommendation of the Rose Review, the nine-month inquiry by Jim Rose, former head of primary at Ofsted.
The Government has since decided that schemes should be approved before use in schools.
The letter was prompted by a TES story exposing plans to require teachers to use only commercial phonics programmes which had been approved by a Government vetting panel.
It is signed by Early Years Curriculum Group members Sally Barnes, Margaret Edgington, Julie Fisher, Janet Moyles, Linda Pound, Dorothy Selleck and Pauline Trudell. All are recognised experts in their field.
Signatories include the teacher unions, national advisers and experts, academics, heads, teachers, parents and governors.
Wendy Scott is a former adviser to the DfES and former chair of the British Association for Early Childhood Education. She claimed that using only approved phonics schemes would boost commercial interests while impoverishing children.
"How do they (ministers) reconcile the insistence on a single, prescribed approach which reduces teachers to technicians with the Rose recommendation that teachers should use professional judgement in relation to their pupils?"
An NUT spokeswoman said: "The insistence on one method ignores the differences and the knowledge that teachers have of the pupils in their classes."