Future Government league tables will no longer take account of pupils' deprivation, ethnicity and other background factors when comparing school exam and test results, The TES has learned.
Unions are warning that the abolition of the contextual value added (CVA) measure without a replacement will further stack the odds against heads and teachers who choose to work in the most deprived areas.
The decision, which will affect the next set of tables due to be published later this year, could even create recruitment problems in schools battling the highest levels of social disadvantage, they said.
Performance tables will continue to show how much progress pupils make compared to their prior levels of attainment, and how many pupils there are with special educational needs in each school.
But all attempts to include this kind of contextual factor in a measure of schools' exam or test performance have now ended.
Martin Ward, Association of School and College Leaders deputy general secretary, said: "This won't help schools in deprived areas to demonstrate they are doing a good job. That is already difficult with all the systems punishing people who take on difficult jobs and rewarding those who have easier ones."
The abolition of CVA ends five years of trying to give schools credit for the nature of the areas they serve and continues a trend of placing more emphasis on their "raw" unadjusted exam results.
Raw results have become more important for Ofsted inspection judgements and have been at the centre of the `National Challenge' floor exam target approach to school improvement begun by Labour and continued under the Coalition.
Ministers first outlined their plans to drop CVA in an education white paper last year, but did not specify a timescale. The TES has now learned that the change will apply from this year.
In the white paper, ministers said: "It is morally wrong to have an attainment measure which entrenches low aspirations for children because of their background.
"We do not think it right to expect pupils eligible for free school meals to make less progress from the same starting point as pupils who are not eligible for free school meals."
But Mary Bousted, general secretary of teaching union the ATL, said: "Schools are to be subjected to the publication of a bewildering array of new data from teacher absence rates to the average salary range.
"It is strange that the Government is taking away the one measure which shows that most schools - given their intake - perform at remarkably similar levels.
"Research shows that most variation is within schools not between them. That is a politically inconvenient truth for governments which invest so much in school effectiveness. The absence of CVA will allow ministers to tell the story they want to."
CVA adds nine contextual factors to a measure of how much value a school has added to their exam results. They also include gender, first language, postcode and whether or not pupils are in local authority care. But some academics have argued the margins for statistical error effectively render CVA meaningless.
Mr Ward admitted the measure had "weaknesses" but said it should be reformed rather than abolished.
The Conservatives revealed they were unhappy with CVA last year while in opposition, but complained when The TES reported it wanted to ditch deprivation from league tables, claiming it would reform or replace the measure.
Ben Slade, head of The Manor - A Foundation School in Cambridge, which has above average CVA said: "It is divisive to remove CVA because it will camouflage coasting schools whether they are outstanding, good or satisfactory."
But a Department for Education spokesman said: "The old CVA score was absolutely meaningless to most parents - and unions know this. We want to make sure that schools' hard work is recognised and understood - the new floor targets include progression specifically to do this."