The Education Decretary will tell the Secondary Heads Association in Newport, Gwent, that the Government will give heads an extra pound;100 million next year, and pound;150m the year after that in direct grants, to meet the cost of moving good teachers up the pay scale.

The pay awards are open to teachers who have passed the new performance threshold, which moves them on to an upper scale where they can earn up to pound;31,000 a year, or up to pound;41,000 if they have management duties.

But heads will also be free to use the cash to move outstanding new teachers faster up the pay scale, to reward advanced skills teachers, or give pay rises to their deputy and assistant heads. Governors will be able to use it to reward the headteaher.

More than 150,000 teachers - around 70 per cent of those eligible - are expected to get the pound;2,000 threshold payment in this first year of the new system.

But the Government admitted this week that payments were delayed and that some teachers will not get the pay rise until May - a year after they filled in their application forms. Heads blame a shortage of external assessors for the delay.

Tory leader William Hague will pledge to reform the performance-pay system if he wins office, simplifying it and giving heads more autonomy.

He is expected to promise not to replace Labour targets and regulations with Conservative ones and to fulfil a long-desired SHA ambition by introducing a national funding formula.

The snag is that the formula will be linked to the Conservatives’ free schools policy, which has yet to win favour among heads.