More than one in seven secondary schools has formally complained about the marking of this year's national tests in English for 625,000 fourteen- year-olds.
English teachers warned last night that many schools would refuse to report test results next year, putting the Government's testing programme in jeopardy once more.
A two-year teachers' boycott of the tests ended last year after ministers promised to improve and streamline the tests in English, maths and science.
But 906 of 6,000 state and independent schools have asked for a review of the English marking, about half questioning marks given to whole classes.
Teachers say bright children have been marked down by inexperienced markers while the less able have been given high marks. External markers were used for the first time this year.
Anne Barnes, general secretary of the National Association for the Teaching of English, said: "This is just the tip of the iceberg. Many schools are saying that they won't report the results next year because they are so pathetically unreliable."
Scripts for this summer's national tests for 11 and 14-year-olds were returned to schools which were able to ask the School Curriculum and Assessment Authority (SCAA) for a review of marking.
There was less discontent with the 11-year-old tests, with 799 of 18,000 primary schools demanding a review in English and 245 and 320 in maths and science respectively. For 14-year-olds, the figures for maths and science were 231 and 256.
Mrs Barnes said some schools had not bothered to complain because they considered the results so "desperately silly". She added: "The tests should be properly marked by professional teachers."
A spokeswoman for the SCAA said the instructions to the exam boards who recruited the markers had been that they should have recent experience of teaching the relevant subject to the relevant age group. "Considering the scale of the operation it went well. There were more problems with English but it is a harder subject to mark."
The authority announced changes to next year's tests to meet the criticisms. For 14-year-olds, borderline cases will be double-checked and markers will receive more training.
t The National Association of Schoolmasters\Union of Women Teachers published a survey of its members in 1,109 primary and 685 secondary schools suggesting the tests had gone more smoothly than critics have suggested.