Orders for the optional English, maths and science tests for eight, nine and 10-year-olds have shot up by a quarter in England.
The Qualifications and Curriculum Authority received 20,000 requests this year compared to 15,000 in 1999. Nearly 2,000 schools which placed their order in Mach and April are still waiting for papers.
Optional tests, introduced in 1997, are used to provide national benchmarks. However, it is believed they will increasingly be used as a measure of pupil progress, one of the criteria for those applying for the pound;2,000 rise.
Employing tests in this way chips away at their voluntary nature, said John Bangs, National Union of Teachers assistant secretary: "Optional no longer means that."