Headteachers could boycott the Government's controversial new reading test for six-year-olds if it is used "as a stick to beat schools with", a union leader warned on Monday.
Russell Hobby, general secretary of the National Association of Head Teachers, (NAHT) 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.
Plans for a reading test were announced by ministers last year, amid fears youngsters with poor reading skills were slipping through the net.
The check, which is taken by pupils at the end of their first year of formal schooling (Year 1), is based on phonics, a system which focuses on sounds rather than recognising whole words, and has been promoted by the Government as the best way to boost reading standards.
Pupils are asked to sound out or decode a series of words, some of which are made up, to test their reading skills.
It will be administered across England for the first time this year.
In his speech to the NAHT's annual conference in Harrogate today, Mr Hobby said: "We fear that the pass rate for the new phonics screening check will be set at an arbitrary high level in order to fuel headlines about children failing to learn to read.
"There is not yet a robust evidence base for any particular pass rate.
"We don't see the need for this screening check - it is inferior to what most schools do already - but if it is to happen it should be used as a genuine diagnostic test, not a stick to beat schools with.
"And if it is used to attack rather than assess, that will be the end of the screening check as far as the NAHT is concerned.
"And we will happily work with our colleagues in other unions like the National Union of Teachers (NUT) to frustrate its further application."
Hobby told delegates that "nobody is saying our education system is perfect", but added it was important to remember the progress that has been made.
"Our schools have got an extra 200,000 children a year reading well, compared with 1995. We are not complacent. But it is pride in what we have achieved so far that will be the springboard to get to 100%."
Speaking before the conference, Mr Hobby gave details on how the NAHT would "frustrate" the reading test.
"What we would do is create our own reading test, administer it, and give it to parents, but we won't report on the data to government," he said.
"Parents will get all the information they need, but the Government won't get any statistics to damn the profession with."
He said that most schools already have their own reading tests in place, which cover more than phonics, and the union will use these to create a standard test.
The NAHT is likely to ballot members for a boycott before any action is taken, Mr Hobby added. His comments come just weeks after the NUT called for a campaign against the test, warning that pupils will be branded as failures at the age of five or six.
Delegates at the NUT's annual conference in Torquay 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 NUT's executive to prepare a campaign, including a boycott, if the test is used towards league tables in the future.