Children should not be tested on made-up words like "voo", "terg" and "bim" to check their reading skills, school leaders warned on Monday.
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.
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.
This includes non-words like "voo", "terg", "bim", "thazz", "spron", "geck", "blan" and "fape" - the National Association of Head Teachers (NAHT), the Association of Teachers and Lecturers (ATL) and the National Union of Teachers (NUT) said.
They are calling on ministers to re-think the test, arguing it will not help to improve pupils' reading, assess whether a child can understand the words they are reading, or let parents know how well their youngster is doing in the subject.
ATL general secretary Dr Mary Bousted said: "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.
"The Government should come clean with parents so that they know the test results will tell them nothing about their child's reading ability or their school's ability to teach reading."
NAHT general secretary Russell Hobby said: "Despite claiming to empower schools, the Government is imposing a narrow test which will actually provide less information than the procedures schools are already using.
"Phonics is an essential part of early literacy, but this approach risks distorting teaching and reduces freedom."
The NAHT has already warned that it could boycott the test, which will be taken by pupils for the first time this week, if it is used "as a stick to beat schools with".
At the union's annual conference last month, Mr 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.
Schools Minister Nick Gibb said: "The unions' position is especially disappointing as many of their members have already told us how this quick check will allow them to identify thousands of children who need extra help to become good readers.
"There is a weight of international evidence which demonstrates that phonics is the most effective way of teaching early reading.
"It is crucial for children to master the basics of reading as early as possible so they can go on to develop a real love of reading.
"Parents want to know exactly how well their children are performing and, if they need help, what their teachers are doing to provide it. This check will help teachers to keep parents fully informed."