Secondaries in affluent areas will lose out on up to pound;80,000 of extra money promised over the next two years by the Chancellor, while schools with disadvantaged pupils could get up to pound;175,000 more, an analysis of how the cash is allocated reveals.

Those with few disadvantaged pupils - and feeder primaries with good results -will get little of the additional funding announced in the Budget, headteachers’ leaders said this week.

But schools with large numbers of difficult pupils will see their annual budgets increase by pound;175,000 more than the average pound;90,000 extra promised.

Headteachers accused the Government of spin and warned that schools already struggling to make ends meet would be the ones to lose out. Class sizes and curriculum options in schools in the worst-funded local authorities would be hit if the funding gap continued to grow, they warned.

Only 15 per cent of the cash will be allocated according to the number of 11 to 16-year-olds at a school. The rest will be according to levels of prior attainment (50 per cent) and deprivation (35 per cent).

Mr Brown said an average secondary would get increases in the school standards grant of pound;50,000 in 2006-07 and pound;40,000 in 2007-08 as the Government attempts to close the funding gap between state and private schools. Under the rules published on the Department for Education and Skills website, an average-sized secondary school of 900 11 to 16-year-olds, with no pupils taking free meals and none who were below the expected standards in English and maths tests at age 11, would receive pound;11,000 over two years.

By contrast, a similar-sized school with three-quarters of pupils eligible for free meals and the same proportion below the expected standard in English and maths would get pound;265,000, according to the Association of School and College Leaders.

Malcolm Trobe, head of Malmesbury school in Wiltshire, who carried out the calculations, said his school will get pound;46,500 extra by 2007-08. It has few pupils on free meals, but a sizeable minority who failed to reach the expected test scores at age 11.

He said: “The Government is concerned about low achievement in areas of high disadvantage and they are prioritising that, but it also needs to address base funding for all schools.

“The risk is that achievement in the less well-funded schools will drop.

Teachers in these schools are working harder to make up the difference, but at some stage this will become unsustainable.”

Roger Hale, head of the 625-pupil Caistor grammar school in north Lincolnshire, said his school would get only about pound;4,500 extra in each of the next two years.

“We were really shocked. Our planned development of popular subjects such as ICT will have to be held back. The worry is that money is increasingly being directed in this way,” he said.

John Dunford, ASCL general secretary, said: “The Budget was cleverly presented to give the impression of massive increases all round, but some schools will find that the reality has nowhere near matched the Chancellor’s rhetoric.”

* jon.slater@tes.co.uk