How pupils' progress should be measured has become a stumbling block in the government's plans for performance-related pay for teachers.

Headteachers, who will be operating the appraisal system which will determine which teachers receive a £2,000 increase, are believed to be pressing for changes in how pupils' improvement can be calculated.

It is expected that teachers will have to prove their ability in eight areas - but the most contentious is reported to be the measurement of pupils' improvement.

But headteachers' unions are pushing for pupils' improvement to be measured against their previous performance - and not against a national target.

Headteachers' leader, David Hart, said that adopting a measurement system based on the individual progress of children - and not measuring them against national benchmarks - could be the key to an agreement.

"This is the key that could unlock the door, or make it very difficult for some teachers to swallow," said Mr Hart, general secretary of the National Association of Head Teachers.

"The only context that makes any sense is in the context of a school's targets."

This was echoed by another headteachers' union leader, John Dunford of the Secondary Heads Association, who argued that "any sensible assessment of teachers' influence over pupils' progress has to be related to those children's own previous best".