The National Association of Head Teachers (NAHT) has warned that it could boycott a new reading test, which will be taken by pupils for the first time this week.
At the union's annual conference last month, NAHT general secretary Russell Hobby said there were fears that the check will be used to "attack rather than assess".
"If this proves to be the case, heads could refuse to administer it, and set their own test instead," he said.
Delegates at the NUT's annual conference in Torquay at Easter passed a resolution arguing that the mandatory testing of phonics is "unnecessary and inappropriate".
They called for concerns to be raised with ministers about the test "at every opportunity" and for the union's executive to prepare a campaign, including a boycott, if the test is used towards league tables in the future.
Phonics should not be the only game in town; it is just one of many equally valid and useful methods of teaching children how to read. Phonics checks for six-year-olds risk doing more damage than good."
– Association of Teachers and Lecturers (ATL) general secretary Dr Mary Bousted
Phonics is an essential part of early literacy, but this approach risks distorting teaching and reduces freedom."
– National Association of Head Teachers (NAHT) general secretary Russell Hobby
As six-year-olds across England prepared to take the Government's controversial new reading test, three teaching unions raised fresh concerns that the check is "flawed" and could do more damage than good.
They suggested that including made-up words will frustrate youngsters who can already read, and confuse those with special educational needs, or for whom English is a second language.
Plans for a reading test were announced by ministers last year, amid fears youngsters with poor reading skills were slipping through the net.