As six-year-olds across England prepared to take the Government's controversial new reading test, three teaching unions raised fresh concerns that the check is "flawed" and could do more damage than good.
They suggested that including made-up words will frustrate youngsters who can already read, and confuse those with special educational needs, or for whom English is a second language.
Plans for a reading test were announced by ministers last year, amid fears youngsters with poor reading skills were slipping through the net.
Christine Blower, general secretary of the National Union of Teachers (NUT), said:
Payment by results is total nonsense. Children are not tins of beans and schools are not factory production lines. Successful schools rely on a collegiate approach and team working.
Performance-related pay (PRP) is not only inappropriate but also divisive. Children and young people differ and class intakes differ from year to year, making it impossible to measure progress in simplistic terms.
PRP will create even more difficulties for schools facing the most challenges - teachers will realise that they will get no thanks for teaching their students but will get more money by going elsewhere.