The Audit Commission has intensified its scrutiny of "coasting" councils through new figures which show underperformance in rural counties and some of the most deprived urban areas in Britain.
The commission's analysis shows some local authorities score twice as well as rival boroughs with similar levels of affluence or deprivation.
This week the commission, a local authority watchdog, attacked the "unambitious average" who never reach their full potential - a message in tune with the Government, the Qualifications and Curriculum Authority, and the Office for Standards in Education.
For example in Camden in north London four in 10 secondary pupils get five GCSE passes at grades A-C. However, Southwark, just across the Thames, manages only half that level. Both rank among the 15 poorest authorities in the country on the basis of the number of pupils eligible for free school dinners.
There is also wide variation at the other end of the social scale. Among the 15 richest authorities, Buckinghamshire hits 60 per cent while in Lincolnshire and Hereford and Worcester only 40 per cent of pupils get up to five good passes - the same level as deprived Camden.
The national average (excluding grant-maintained schools, which the Audit Commission does not examine) is 38 per cent.
The figures appear in Local Authority Performance Indicators, published annually by the commission since 1994. This year, for the first time, educational statistics have been allotted a booklet and publication date of their own.
The wide variation in school and local authority performance has been picked out by the Government as evidence that there is much room for improvement in the education system.
"This sort of variation must raise very big questions about the claim that deprivation is a barrier to educational achievement," said Paul Vevers from the Audit Commission.
He attacked "the unambitious average, a group of councils who solidly achieve the average but never more from it, when similar councils do better for the same or less cost."
The disparities in achievement have already been worked over by the Qualifications and Curriculum Authority and, most recently, by the Office for Standards in Education.
Both the QCA's Benchmarks and the OFSTED's Performance and Assessment profiles allow comparisons between the results of similar schools.
But the exercise also shows a clear correlation between poverty and underachievement: 35 per cent of metropolitan pupils get five good GCSEs, compared with 45 per cent of county councils.
"There is a relationship between academic achievement and the level of deprivation," says the report. "However I it is vital that schools in those deprived areas which are performing relatively poorly are helped to learn from the experience of those which are equally deprived and are doing better."
Among the metropolitan councils, Solihull and Stockport score highly at around 50 per cent while Bradford and Knowsley struggle at just over 20 per cent.
The picture among the new unitary authorities is scarcely different with York and North Somerset (50) at the top and Hull (21) at the bottom. County councils are much more closely grouped around a 45 per cent average, particularly when Buckinghamshire (60 per cent and well ahead) is removed.