Children will be branded failures at the age of five by the Government's new reading test, teachers have warned.
The National Union of Teachers (NUT) raised concerns that the check will "demotivate" children, particularly bright youngsters who are good at reading.
It warned that the test militates against clever pupils and could lead to children being drilled in how to pronounce "made-up" words.
A poll conducted by the NUT found that two thirds of teachers (66%) think the test is unnecessary, and 67% believe it is a waste of money.
The union is due to debate a resolution at its annual conference in Torquay today, raising the prospect of a boycott if results from the test are used in league tables in the future.
Ministers announced plans for the 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 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.
But the NUT argued that the use of so-called "pseudo words" will be confusing to bright children and may result in teachers teaching youngsters "to not read".
The union's general secretary, Christine Blower, said: "The fact is there will be a point when children are deemed to have failed it, and we think that's a very poor start in education."
Hazel Danson, a phonics teacher and chairman of the NUT's education committee, said reading is "so much more than just decoding a text. They might as well be decoding a page of French.
"What we have been advocating is very much reading for pleasure, with, along the side, the skills you need to read."
Ms Danson said that in the pilot of the test, which was carried out last year, some bright children struggled as they were trying to make real words out of made-up ones, and failing as a result.
"One of the pseudo words is 'osk', she said.
"Visually, it looks very similar to the word 'ask' and on the test there were children with more developed visual reading skills who didn't get that word right. There were a number of other words like that that they read incorrectly because they were applying what I would say is a higher order, or just a different, reading skill."
Ms Danson added: "It was confusing them and they were scoring badly because you only get one go at the word and you have to write down exactly what the child says."
There is "no discretion" for teachers, she said.
Ms Blower said that the check "militates against children who have already got those higher order reading skills".
"But then there's also a very odd perverse incentive in that perhaps what we'll see is teachers drilling kids in non-words.
Because if you know that you're a better, or more advanced, or more able, reader you might try and make a word out of a word that's a non-word.
"Teachers will have a tendency to say 'well, let's practice lots of non-words, so when you see a non-word you don't try and make them be words'."
"How stupid is that?" she added.
Ms Blower said that if, at some stage, the test results are used in league tables "you would have people doing the exact opposite of what you want them to do. You would be teaching them (children) to not read, essentially".
She added: "Our members are saying five is too young to fail.
"When reading is essential to being able to work with the rest of the curriculum, why would you want to do something that would potentially demotivate not only the children who might have a lot of difficulty with the test because maybe they haven't reached that level, but also the kids who are actually beyond that who then fail it because they are trying to bring skills to bear which are not useful to being able to do the test?"
The NUT's motion opposes the Government's claim that the check will raise reading standards and says each child learns to read in a different way.
It says: "Conference asserts that the introduction of statutory testing of phonics for all Year One pupils is unnecessary and inappropriate."
It calls on the NUT's executive to raise the union's concerns with government and to highlight their worries to parents and the public.
An amendment also calls for the union to "prepare a campaign, including a boycott, should the Year One phonics check be used to contribute to any kind of league tables".
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."
On Saturday the NUT and NASUWT unions voted for further industrial action, including strikes, in a dispute with Government over pensions, pay and job losses.