Average class sizes in private schools in the UK at primary and secondary levels are just above 10, according to the Organisation for Economic Co-operation and Development, the international think-tank based in Paris.
By contrast, the average state primary class size of 23.6 pupils is well above the OECD average of 21.5 and the European average of 20. State secondary class sizes of 24.9 are also above both the OECD (23.8) and EU (22.5) averages.
In March Gordon Brown, the Chancellor, used the Budget to draw attention to these differences and promised to close the funding gap. He said: "We know the educational benefits of more individual attention, small group teaching and tutoring, and that they are easier to get where the overall teacher pupil ratio is low."
Steve Sinnott, general secretary of the National Union of Teachers, said:
"This is a shocking disparity. The Government should concentrate on meeting Gordon Brown's promise rather than going down the blind alley of trust schools and changing structures."
Alan Johnson, the Education Secretary, used a keynote speech this week to call for state schools to emulate the best features of their private neighbours.
In its Education at a glance report, the OECD warned that Britain faces a "serious bottleneck" of teenagers who fail to get good GCSEs. Slow progress at that level over recent decades meant a limited number of teenagers went on to take university courses, while other countries improved much faster, it said. As a result, the UK has slipped down international league tables for the number of students leaving school and university with key qualifications.
The report, which covers the period up to 2004, analysed educational progress in 30 developed countries. It said: "While upper secondary attainment in the UK has increased over the generations, many other countries have recorded faster progress."
Andreas Schleicher, the OECD's head of analysis, said UK teenagers who leave school without five GCSE C grades had more to lose than their counterparts abroad: "The worrying thing is at the low end: the penalties for those without baseline qualifications. Pressure from globalisation is not likely to decrease and the UK is more vulnerable because of its open labour market."
Ministers welcomed the report's recognition that education spending in the UK had increased faster than elsewhere. As a percentage of national income it has almost reached the OECD average.
The report also showed that Britain had improved its position in getting more teenagers to stay on in education or work-related training after the age of 16.
Details of "Education at a glance" are available from www.oecd.org