The UK has 24.5 students in each state classroom, compared with the average of 21.5 pupils, according to a study by the Organisation for Economic Co-operation and Development (OECD).
Only Japan, Korea and Turkey have larger classes than the UK.
This is despite the fact that the UK spends more per primary school pupil than the OECD average.
The report noted that "unusually" the UK has lower class sizes at secondary level, unlike most other OECD countries.
On average, the UK has 22.4 students compared to the OECD average of 24.
In contrast to primary schools, the UK spends less per student at secondary level than the average.
The report found dramatic differences between the numbers of pupils in state and private schools.
It said: "The UK stands out as the OECD country with the largest difference in class sizes between public and private institutions: in primary education, there are 13 pupils more per classroom in public institutions than in private ones, whereas on average across OECD countries, class sizes do not differ between public and private institutions by more than one or two students per class."
Another report released today by think tank Civitas found that at infant school level, small classes are essential to young children's educational achievement
It noted that current class sizes are too big and must be cut to "maximise learning opportunities among infants".
It said evidence shows that classes of 20 pupils or under for the first three years of school "produce long-term benefits for literacy and numeracy, especially for low achievers".
Authors of the annual Education At A Glance report, analysed the educational progress of more than 30 countries.
They found that while the UK is performing strongly in some areas, it is still lagging behind in others.
For example, the report found that the UK invests more money per child in pre-school education than every other country except Austria, Iceland and the United States.
It also has one of the highest participation rates for young children, with 90 per cent of those under four in pre-school programmes.
But the numbers of teenagers staying on post-16 is still relatively low, with just 69.7 per cent of 15 to 19-year-olds remaining in education, well below the OECD average of 81.5 per cent.
Only Mexico and Turkey had lower rates.
The report found that there has been massive private investment in education in the UK in recent years.
It said: "Taking all levels of education together, private spending in the UK rose faster than public spending between 2000 and 2005."
In primary and lower secondary schools (up to the age of 13) the share of funding from private sources has increased from 11.3 per cent to 17 per cent - more than any other nation except the Slovak Republic.
At university level, there has been a 53 per cent increase in private spending, meaning that in total 33.1 per cent of higher education funding now comes from private sources.
In contrast, public spending on higher education rose by 48 per cent in the same timeframe - 2000 to 2005.
While the UK continues to produce good graduation rates at university level, in relative terms it is beginning to drop behind.
In 2000, the UK had the fourth highest graduation rate for people gaining first degrees, at 37 per cent - well above the OECD average of 28 per cent.
By 2006, that rate in the UK had risen to 39 per cent but the OECD average had risen to 37 per cent, with 11 other nations now showing higher rates than the UK.
This is due to some countries massively expanding their education systems in the last few decades.
The report notes that current rates of participation suggest that more countries, like Korea, are likely to surpass UK graduation rates.
A spokesman for the Department for Children, Schools and Families said class sizes had become steadily smaller in the past 10 years, with better adult to pupil ratios.
He said: "In 1997 there were 17 pupils per adult at primary level, now there are 12.
He added: "The OECD itself has praised England for lowering class sizes, along with consistent and strategically spent money to attract better quality teachers."
Minister for Lifelong Learning, Further and Higher Education Bill Rammell said: "The OECD figures highlight the high quality of our higher education system and UK graduates still enjoy a better return on their investment than most OECD countries.
"This is at a time when record numbers of students from all social classes are choosing to go into higher education and reap the benefits this brings."
Schools Minister Andrew Adonis said: "I am delighted that we can point to positive findings across the education system as a result of this report.
"From early years to adult skills, the OECD has confirmed that we are targeting investment well and sticking with policies that work.
"We are at the beginning of an exciting period of change that I am sure will see even further success."
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