One in five schools in England fails to give pupils a decent education in maths and English, although the trend is improving, exam league tables showed on Thursday.
Some 639 state-funded schools last year failed to meet the government target of 30 percent of their students achieving a good grade - at least a C - in five GCSEs including English and maths.
Prime Minister Gordon Brown said in November he wanted all schools to reach this level by 2012 in the exams taken by 15 and 16-year-olds.
He said those that failed to hit the target faced closure or federation with another school.
Schools Minister Jim Knight said 150 more schools had made the required grade in Thursday's tables than the previous year.
"The vast majority are improving year on year and those at the lowest level are receiving intensive support."
He said improvement over the past decade was excellent - in 1997 only half of schools reached the target.
The opposition Conservatives said the tables showed that more than 500,000 pupils were at schools the prime minister regarded as failing.
Less than half of all students - 46.0 percent - made the target grades, a level the government aims to boost to 53 percent by 2011.
Sutton, in south London, was the best performing local authority with 64.7 percent of pupils gaining the required GCSE passes. Knowsley, in Merseyside, was the worst with 26.7 percent.
Teaching unions urged the government to scrap the tables and follow the lead of devolved administrations in Scotland, Wales and Northern Ireland where no school rankings are published.
"The GCSE league tables are of no educational value, undermine collaboration between schools, and tell parents little about schools or their child's learning," said Mary Bousted, general secretary of the Association of Teachers and Lecturers.
But Knight said parents had a right to know how their school was doing and the tables would stay.
Leading independent schools such as Eton and Winchester found themselves at the bottom of the tables because the government does not recognise the "harder" International GCSE exams (IGCSEs) their pupils take in maths and English.
Knight said the tables would be "meaningless and chaotic" if they included non-accredited qualifications such as IGCSEs, which do not follow the national curriculum.
