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.
More top news
The world’s coral reefs are struggling. Scientists have declared that reefs across the world are facing a “global bleaching event”.
Some of us might be turning the heating on this week as a wind chill and low temperatures sneak in.
Facebook is to test a set of emoticons to add to the like button giving users more choice to react to their friends' status updates.