More than 100 schools will drop at least 1,000 places in England's GCSE league tables as ministers change the method by which the rankings are calculated.
Hundreds of other schools will slip down the tables as ministers toughen up their method for ranking secondaries to place more emphasis on pupils'
performance in English and maths from 2007.
In Wales, the Assembly government set targets for GCSE performance in the three core subjects of English or Welsh, maths and science in its 2001 programme for education and lifelong learning, The Learning Country.
However, last summer only 38 per cent of pupils achieved passes in the three core subjects at grades A*-C, compared with Learning Country targets of between 40 to 60 per cent by 2004 and between 50-70 per cent by 2007.
The pass rate for five or more good GCSEs was 52 per cent.
The Assembly government does not publish performance tables for schools, but has released secondary results under the Freedom of Information Act. In 2005, Welsh schools' results ranged from 6 to 80 per cent of GCSE candidates achieving the three-subject core indicator.
In England, the new two-subject measure will be used in official tables from 2007, as ministers address concerns that schools have been able to claim apparently impressive results despite poor performance in core subjects.
It replaces the system which assesses the proportion of pupils achieving five or more GCSEs or vocational equivalent at grade C or better.
The TES compiled figures comparing secondaries' performance based on the old and new measures. Nationally, the average rate of performance in 2005 falls from 56 to 44 per cent under the new measure. Those sliding down tables risk losing parental support and the funding that goes with it.
English and maths teachers' associations are warning that their members face more pressure because of the new weight given to their subjects. The move will also greatly reduce the impact of general NVQs on schools'
rankings, following controversy about their contribution to headline scores.
Among the losers are academies. The biggest loser among state secondaries, Grange school, Oldham, drops more than 2,500 positions when the rankings of nearly 4,000 schools for this year's GCSEs are changed from the old measure to the new.