Tens of thousands of teachers appear to have been needlessly ruling themselves out of pay rises worth thousands of pounds.
The latest official figures show that nearly half of teachers eligible to apply for salary boosts of up to #163;4,720 have not been doing so, even though more than 90 per cent of those who did apply for the extra money were successful.
The system has been changed this academic year to make qualification for the upper pay scale less bureaucratic and time-consuming.
But classroom teacher unions fear that pressure from heads could still put teachers off going for the money - a suggestion described as "bizarre" by one heads' leader.
The upper pay scale has been in place for nearly a decade and was originally supposed to represent the introduction of performance-related pay for experienced teachers.
But in 2001, when the first batch of teachers were able to apply to reach the new pay scale, 78 per cent of those eligible applied and the vast majority were successful.
Since then the proportion of those at the top of the main pay scale prepared even to try for the progression - worth a salary hike from #163;35,568 to #163;40,288 in inner London - has dropped markedly.
In 200809, the latest year for which figures are available, only 47 per cent of eligible primary teachers and 60 per cent of eligible secondary teachers went for the money.
Martin Freedman, head of pay and conditions at the Association of Teachers and Lecturers, noted that there had been a particular drop in applications since 200405, when a system of external checks on heads' decisions to move to the upper pay scale was dropped.
John Bangs, head of education at teaching union the NUT, said the end of external validation was a "big mistake". "It means that sometimes the process is self-selective," he said. "Some teachers might think their heads won't give them a fair run or that there is not enough money in the budget, and don't apply."
Mr Freedman said: "We have anecdotal information that some teachers have been told, 'Don't bother, you won't get through,' and that is a concern. It may be that some feel it is additional hassle they can do without. If they are eligible they should apply because it is about their career progression and development."
Since September teachers no longer have to provide their own evidence to show they meet the threshold standards and can base applications on performance management reviews. But Mr Freedman said there was a danger that heads might be tempted to alter these for certain teachers to keep wage bills down.
The TES understands that there was union pressure within the Government's social partnership to prevent heads from doing anything but rubber-stamp the performance management decisions of department heads, but this was rejected.
John Dunford, general secretary of the Association of School and College Leaders, said: "It is a fair, open system agreed by (classroom teacher) unions. If teachers choose not to apply, it doesn't make sense to blame the head."