A consultation paper produced by the panel of head teachers that advises on the 11-plus in Kent warns that the English tests are unreliable and poor indicators of ability.
Anti-selection campaigners said yesterday that the document, which lists the "acknowledged advantages and disadvantages" of the tests, was proof that the grammar school system was unfair. But defenders of grammar schools said the system enjoyed widespread public confidence.
Education officers and headteachers acknowledged that the English tests had long been criticised by some heads, but insisted the 11-plus exams were only one part of the selection process.
The revelation will reignite the debate over the remaining 166 grammar schools, which face abolition if parents vote to end selection.
The anti-grammar campaign is at its most advanced in Kent where campaigners hope to trigger a ballot of parents possibly later this year.
The consultation paper, sent to all Kent head teachers before Christmas, was drawn up as part of an exercise to revamp the 11-plus next year. It warns that comprehension tests used as part of the 11-plus are "not reliable or discriminatory" and "give no correlation between ability and achievement". Marks for grammar and punctuation are "susceptible to coaching".
The paper praises the mathematics test, which forms the second part of the 11-plus, as "extremely reliable" and praises IQ-style verbal and non- verbal reasoning tests used by some of the county's grant-maintained grammar schools, which control their own admissions policies.
Half of Kent's 16,000 11-year-olds will take the 11-plus next week in the hope of becoming one of about 4,000 expected to gain a place at the county's 31 grammar schools.
Yesterday Martin Frey, spokesman for the Stop the Eleven Plus pressure group, said: "We regard it as completely wrong that life-changing decisions for young children should be taken on an indicator which is known to be flawed."
Eric Hammond, the former electricians' leader and chairman of the pro- grammar Save Kent Schools campaign, said: "All the tests of opinion have put support for our selective system in the Nineties. Support is of east European proportions. It's clear that people do have confidence in the system."
Each year the Kent 11-plus is drawn up from a bank of questions set by the National Foundation for Educational Research, which also supplies questions to several other authorities that retain grammar schools, such as Bexley.
The foundation insisted that Kent's test were accurate and consistent. Chris Whetton, the assistant director, said: "We analyse the tests for Kent and our analysis of their data showed they have high levels of reliability. We don't understand their criticisms."