The performance pay system, due to come into force on Saturday, will offer a �2,000 increase for teachers who pass a quality "threshold", which is to be determined by an appraisal.

The government says it is reasonable to expect line managers to contribute to assessments

But as well as head teachers and external assessors, the union leader is concerned that other members of staff will be expected to comment on the suitability of a colleague for an increase.

"It's a form of snooping. If you apply for a pay rise and your colleague has to give an assessment of you - well, if you get the �2,000 he'll want you to buy him a pint," said Mr McAvoy.

Destroy teamwork

"But if you don't get it, it could be because the external assessor didn't like you but your belief will be that your colleague may have said something bad about you. Whether he shares his chalk with you in the future would be debatable.

"It will destroy professional teamwork. Teachers will be constantly suspicious of each other," said the union leader.

But the government has rejected this accusation, saying that it was reasonable to expect line managers, such as heads of department, to make assessments and that there was no suggestion of teachers "snooping" on colleagues.

In an attempt to block the introduction of performance pay the union is seeking a judicial review - and has repeatedly threatened strike action.

But the union has already conceded that its members might apply for the performance payments and that there will not be a boycott of appraisals or the implementation of the merit pay system.

The union's legal action is based on the claim that late changes to the legislation that introduces performance pay have not allowed the required period of consultation.