“Key stage 2 test results really are a pathetic way of asking parents to make a judgment about schools.”

No, that is not a speaker at the annual conference of the National Union of Teachers. It is John Morris, general secretary of the Incorporated Association of Preparatory Schools, which represents more than 500 prep schools.

The popularity of the national tests for 11-year-olds has plummeted among private primaries over concerns about league tables and their impact on the curriculum. In the past three years, one in three has dropped the tests.

Only 37 per cent now use them.

Although private school results do not appear in local authority tables, the scores are available from the Department for Education and Skills, and schools fear they are being used to make comparisons.

Mr Morris said: “You can’t characterise a school by its results. League tables undervalue the breadth of provision at our schools. We take academic success as a given and are in the fortunate position of being able to offer a wide range of other activities.”

Pupils often continued at prep school until 13, which made testing at 11 “plainly silly”, he added.

Mr Morris said the association objected to league tables despite prep schools scoring well above the national average at key stage 2. Ninety per cent of pupils last year achieved level 4 or above in all tests and around two-thirds obtained level 5 in English, maths and science. The level 5 results were at least 30 per cent higher in IAPS schools than the national averages, he said.

Rowland Constantine, chairman of the association and headteacher of the Pounds 10,000-a-year all-boys New Beacon prep school in Sevenoaks, Kent, said: “Schools are voting with their feet. The majority have now come out against the tests.

“All along we have made the point that testing has a negative impact on schools. It distorts the curriculum and encourages teaching to the test. We did not have any complaints when we stopped using the tests two years ago.

“The English tests are quite good and have a purpose, but the maths and science are not of the same quality. The effect they have on schools is out of all proportion to their value.”

Private schools do not have to take part in national testing at 7, 11 and 14. But Mr Morris said: “Some will continue to use them internally for their own purposes, but not have the results validated by the QCA. This will stop their results being published.”

He said it was up to individual schools to decide whether to use the tests and that the association was not urging a boycott.