A teachers' union has voted to ballot for a boycott of a new phonics reading test if its results are used in league tables.
The new "phonics check" for five and six-year-olds is due to begin in England's schools this June.
The government says it will help identify children who need extra help.
At its annual conference, the National Union of Teachers (NUT) attacked the test, saying it risked making failures of five-year-olds.
There had been calls for a ballot on a boycott of the tests in general, but they were dropped.
Instead, union delegates agreed to campaign against the tests and to ballot for a boycott should the check be used "in any kind of league table".
This year's tests will not be affected.
The government has said the results will not be put in to league tables but teaching unions fear they could be in the future.
Results from Sats taken by 11 year olds are used by media organisations to draw up primary school league tables, which many within teaching unions, loathe.
Some fear the phonics reading test may be used in the same way in the future.
Union leaders say the test will not tell teachers anything new.
The NUT says nine out of 10 of its members who took part in an online survey said the check would give teachers no extra information on their pupils.
Its general secretary, Christine Blower, said the check would not be good for pupils: "Five years old is too young to fail. They will know that they have not got it right and we think that will be demoralising".
The reading checks will be run in England's schools in June and are expected to take five to 10 minutes.
Children at the end of Year 1 will be asked to read 20 words and 20 "made-up words" such as "zog" or "vot" to their teachers.
The check is aimed at measuring whether pupils have a good understanding of phonics - the sounds of letters and groups of letters - which the government says is the key to helping children to read.
In reality, all schools use phonics to teach children to read, but the government says this should be done more systematically.
Teachers say they should be trusted to use their judgement to use a range of techniques suited to their pupils.
John Holmes, from the union's executive, said: "This does absolutely nothing to inform or raise reading standards.
"Pilots show it will label two-thirds of children as failures at five or six."
The main conference motion called for the union to continue to raise fears about the test with the government and to tell parents about them.
Jennie Harper, a primary teacher from Croydon, told delegates: "What concerns me most is that the message being sent to parents is that the teaching of phonics is a magic reading medicine that enables all children to read.
"All children are different, and there's no magic one-size-fits-all way of teaching children anything."
Speaking earlier, a spokeswoman for the Department for Education said: "Standards of reading need to rise. At the moment around one in six children leaves primary school unable to read to the level we expect.
"The new check is based on synthetic phonics, a method internationally proven to get results."
She added that the check would stop thousands of six-year-olds "slipping through the net" and lead to their getting the help they needed.
At its conference the NUT also raised the prospect of teachers in England refusing to co-operate with Ofsted inspections.
It passed a resolution with an amendment which called on the union to "consider whether non co-operation with inspection arrangements could be pursued as a workable action strategy to defend members and to support our campaign".
The conference heard there were already teachers in Northern Ireland who were refusing to co-operate with their schools inspectorate.