The NASUWT said heads were using various unreasonable excuses to prevent staff from progressing to the upper pay scale. It said a teacher at an inner-London primary, which it would not identify, had been blocked from progressing because he was not a “team player” and had not attended the Christmas party.

Other reasons given in the past year include lack of money in the school budget and staff not running after-school clubs and assemblies.

But Mick Brookes, general secretary of the National Association of Head Teachers, said the criticism of heads “does not hold water”.

“I would really like to see the basis for these claims,” he said. “Heads recognise that the school budget is not an excuse for stopping pay rises. These claims are part of an anti-management culture.”

Figures suggest thousands of teachers are being paid less money than they could earn by not asking to be assessed for the upper pay spine.

Of those eligible to join the upper pay grades, just 50 per cent of primary teachers and 60 per cent of secondary teachers volunteer for the assessment.

But 95 per cent of the teachers that do apply are successful. In inner London, that would result in a pay rise of almost pound;4,000, up to an annual salary of more than pound;37,800.

Kathy Duggan, a member of the NASUWT’s national executive, said some teachers did not know what they were entitled to. The smaller size of primary schools could make the problem worse, she added.

“Primary schools can behave like extended families, and some of them are dysfunctional,” she said.

“Many tactics can be employed to prevent a teacher volunteering for assessment. Some old favourites can be easily combated, but others are less obvious and more insidious.”