The biggest teachers' union has told the High Court the new performance-related pay system in England and Wales will cause division in the classroom.

The National Union of Teachers has successfully applied for judicial review of an order introducing new professional duties for teachers.

It says this involves an obligation to "snoop" on colleagues as part of the appraisal process for those seeking to pass the pay "threshold" to get a �2,000 rise.

The government says the intention is that the change applies only to departmental heads.

At issue is an amendment to teachers' existing responsibilities, contained in The Education (School Teachers' Pay and Conditions) (No. 2) Order 2000.

'Highly divisive'

To their other duties this adds "assisting the head teacher in carrying out threshold assessments of other teachers".

Gavin Millar QC, for the NUT, argued that the Education Secretary, David Blunkett, had acted outside his powers.

He told Mr Justice Jackson the union particularly feared that problems were likely to arise if a fellow teacher was asked to assess how pupils of a colleague were progressing.

"It needs little imagination to appreciate that is highly divisive and contains potential for conflict between teachers," he said.

Conflict could occur between head teachers and staff members reluctant to assist them.

It was traditional that "when a classroom door is closed colleagues don't step through it and start assessing the performance of that teacher".

'Blanket order'

Teachers had never before been required to participate in the processes which determined the pay of professional colleagues.

"One could readily imagine that if an applicant failed to cross the threshold standard because of the intervention of a fellow classroom teacher, there is considerable potential for division and conflict in that sort of situation," he said.

It had been suggested by the government that the duty of assessment would be restricted to line managers, or school staff with management responsibility.

But the order did not say that.

"The duty is framed in blanket terms of teachers liable to be asked, and if asked, obliged to assist in the assessment," he said.

There were also real concerns about the potential extra demands the new scheme would have on the workload of "already administratively overburdened head teachers and teachers".

Mr Millar said the union believed there should now be proper consultation and "thorough and careful consideration" of the issues.

The hearing was due to continue on Wednesday.