Teachers have called for a campaign against the government's new reading test, including a possible boycott, as it warned pupils will be labelled as failures.
Delegates at the NUT's annual conference in Torquay passed a resolution arguing that the mandatory testing of phonics is "unnecessary and inappropriate".
The union argued that the government's policy of promoting phonics will send a message to schools and parents that other aspects of reading are less important.
It 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.
Ministers announced plans for the reading test last year amid fears children with poor reading skills were slipping through the net
The test, which is taken by pupils at the end of Year 1, the first year of compulsory schooling, 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.
Speaking during the debate, John Holmes, of the NUT's executive, said: "This Year 1 phonics check is just one more example which does absolutely nothing to inform or to raise reading standards. Indeed, the pilots have shown that it will label two thirds of children as failures, at the age of five or six."
Teachers know how well children can read, how they are progressing and the best ways of helping them. Mr Holmes said.
He said that the test could become as high-stakes as national curriculum tests, known as SATS, which are taken by pupils at the end of primary school.
"There's a risk of it becoming the only methodology as teachers fear the consequences if they don't follow it to the letter," he said.
Jennie Harper, a primary teacher from Croydon, told delegates: "What we do not yet know is the effect this test is going to have on children, teachers, schools and parents."
She added: "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. And if it doesn't work, then the fault lies with the way it's been taught, with me, or within the child themselves.
"All children are different, and there's no magic one-size fits all way of teaching children anything. "
She said that some children are going to fail.
"They have recognised needs that impact on their ability to deal with the complexities of the English language system, and the phonics system," Ms Harper said.
"But they are bright, sparky children that are a joy to teach and they are going to be described as failures by a government who doesn't care about abusing and upsetting small children in the name of testing."
Ms Harper also said that the test includes "nonsense words", adding "it is a true reflection of the testing culture that we are now in that we are now testing nonsense."
A Department for Education spokesman said: "We have been clear that the results for the reading check will not be published in league tables. Schools will be required to tell parents their own child's results.
"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, and one in 10 boys leaves able to read no better than a seven-year-old. These children go on to struggle at secondary school and beyond.
"The new check is based on synthetic phonics, a method internationally proven to get results. The evidence from the pilot carried out last year is clear - thousands of six-year-olds, who would otherwise slip through the net, will get the extra reading help they need to become good readers, to flourish at secondary school, and to enjoy a lifetime's love of reading."
The NUT also today threatened to refuse to co-operate with Ofsted inspections in the future.
It passed a resolution containing 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".
There are already teachers in Northern Ireland who are refusing to co-operate with their schools inspectorate, the conference heard.
Speaking during the debate on the amendment, Martin Powell-Davies from Lewisham in south London said: "I'm sure there are lots of us who have considered whether we could boycott Ofsted, whether we could have non co-operation, and perhaps that's going to be a lot to ask people to do."
If there are teachers that are already doing it then "it has to be something that we very strongly consider".
He said that when the amendment had been discussed there had been excitement among members of "the thought that you might just be able to tell that inspector 'class, stop what you're doing we've got an unwelcome visitor and we need them to leave'".
Mr Powell-Davies added: "I do want to stress that the wording is not instructing the executive to say that this is something that we are definitely going to do. What it's asking the executive is to consider it, to strongly consider it."
Keith Williamson from Kirklees, who proposed the main motion, said: "Surely we should look at whether non co-operation could be pursued. Let's face it, the strategies we have used so far have failed."
"Does anyone believe that Ofsted today is better than it was in previous versions?" he added.
The motion raised concerns about changes to inspections, which will see weaker schools inspected more frequently.
It says: "The new inspection arrangements will not reduce the stress and workload created by the inspection regime.
"Conference also notes that the linkage of test and examinations results with Ofsted grades will continue to penalise those schools serving the most disadvantaged areas."
The motion calls for the union's executive to "reinvigorate the campaign for the abolition of Ofsted" and monitor the effect on inspections on schools.
An Ofsted spokesman said: "Recent independent studies have shown the positive impact inspection can have on school performance. Head teachers also regularly tell us following their inspection that it was a positive experience, with more than nine out of 10 saying it will help the school to improve.
"In addition parents value our work and rely on our reports; Ofsted's website is one of the most used in the public sector.
"Ofsted inspects schools proportionately so that our resources are focused on less successful schools where inspection can highlight those areas of individual schools' work which most need improvement.
"We take into account the school's context and do not expect the impossible but there are hundreds of truly outstanding schools serving the most deprived communities, if they can do it, then others can too."
A Department for Education spokeswoman said: "Parents and taxpayers would rightly be asking questions if NUT members prevented them from a full understanding of how well schools are doing. We want to recognise and reward schools that have demonstrated the strongest performance, and concentrate inspection where it is most needed."