Headteachers say a "substantial question mark" remains over the government's plan to introduce performance-related pay for teachers this year.

The National Association of Head Teachers says its members need more time to prepare for an appraisal system for classroom teachers which will determine levels of pay.

The headteachers' union says it is "extremely unrealistic" for the government to press ahead with its proposals to introduce "the world's biggest performance-related pay system" in September 1999.

Instead the NAHT, in its response to the government's Green Paper on reforming pay, says it would "infinitely prefer" to wait until September 2000 before introducing the new pay structures.

Although acknowledging that "no change is not an option", the union calls for a slower pace of reform, with more time for headteachers to be trained and prepared for the new responsibilities.

Heads want assurances any awards will be paid for in full

Headteachers are concerned that running the appraisal systems will mean an increased workload and that more paperwork will be generated.

The union is also seeking assurances that there will be funding for whatever increases are created by the appraisal system - "even if it means spending more than £1bn".

Under the government's proposals classroom teachers who are appraised and successfully cross an ability "threshold" will receive a 10% pay rise. The NAHT warns that "it would be disastrous if teachers who go through the threshold cannot be so supported because the money is not there".

David Hart wants the government to invest cash as well as "political capital"

The union has also outlined its estimates of the cost of running the performance-related pay system, claiming that the annual bill to schools will be £132.5m. In the first year, the union says there will be extra costs, estimated at £77m, as large numbers of teachers apply for appraisals and increases.

There will also be a bill for training teachers and headteachers for the new appraisal arrangements, the union says, putting the cost at £46m.

The General Secretary of the NAHT, David Hart, said: "The government has invested a great deal of political capital in a brand new pay structure for teachers. It must invest an equal amount of money in making it work.

"Everyone knows that the system cannot operate without heads."