David Blunkett, Secretary of State for Education, said the increase of 1.1 per cent in the proportion of entries achieving grades A*-C - up to 55.8 per cent - showed good progress towards national targets. The rise is the biggest for five years. But 100,000 entries - 2 per cent - did not merit even the lowest of the eight grades. The figure was down 0.3 percentage points on last year.
Heads blamed the gap on examination league tables: rating schools according to the proportion of teenagers achieving five high-grade GCSEs encouraged them to neglect those of lower ability.
John Dunford, general secretary of the Secondary Heads Association, said: "The league tables ... encourage schools to concentrate on a very small part of the cohort at the C-D borderline. Any sensible measure of school performance should reflect performance across the full ability range."
David Hart, the general secretary of the National Association of Head Teachers, said: "The fact that there is a smaller increase in the success rate lower down the scale demonstrates that there is a polarisation between the educational haves and have-nots."
A government source said the claim was "utter rubbish"; there was no evidence schools were abandoning the slowest pupils to meet national targets for top grades. "These claims were made last year. Then, for the first time in four or five years, we had a reduction of 1 per cent in the numbers leaving without a single pass."
While the Conservatives called for an inquiry into whether standards were slipping, exam boards said they were as rigorous as ever and the Government pointed to studies set up by the Conservatives that showed GCSE was as challenging as ever.
Professor Alan Smithers, of Liverpool University, said that one reason for the improvement in the proportion getting the higher grades might be that schools were entering their weaker pupils for short-course GCSEs, now in their second year, and the Certificate of Achievement, designed for less able pupils.
Results for short courses, which account for 4 per cent of entries, were significantly below those for full courses.
Mr Blunkett said: "We have begun to offer 14 to 16-year-olds a more vocational route, with greater work-related learning. I believe this will help us to build on the welcome reduction we had last year in the number of young people leaving school without any qualifications."