UK universities face a "serious bottleneck" in finding enough properly qualified students for courses, an international comparison suggests.
Many countries are expanding higher education faster than the UK
The OECD said the UK had fallen from second to ninth in its table showing the proportion of people with degrees between 2000 and 2004.
While more people stayed in education after the age of 16, there were too few to meet universities' future demands.
But the government said UK education was showing "continued improvement".
Ministers want "towards half" of all young people in England to enter higher education by 2010.
The OECD - Office for Economic Co-operation and Development - compared education systems in more than 30 developed countries.
It found the UK had an "impressive" growth in higher education, with 39% of young people having completed degrees or equivalent courses.
The number of people starting courses had risen by 20% between 1995 and 2003.
However, the average increase in all countries the OECD looked at was almost twice that, at 38%.
The UK may soon be suffering from an "insufficient pool of individuals who are suitably qualified" for university, the OECD said.
The "relatively large proportion" of pupils leaving school without qualifications had been "manageable" in the past, but with greater international competition this was no longer the case.
In 2003, the UK had the 13th highest proportion of 55 to 64-year-olds with school qualifications, such as O-levels.
Among 25 to 34-year-olds it came 23rd.
By comparison, South Korea had the 24th highest proportion of 55 to 64-year-olds with school qualifications.
Among the younger group, it came first - suggesting large-scale investment was rapidly creating a better qualified workforce.
The head of indicators and analysis at the OECD, Andreas Schleicher, said a similar improvement was happening elsewhere, in countries such as China.
He added: "China and India are no longer just competing on the low-skills market but are competing with higher qualifications."
England's higher education minister, Bill Rammell, said: "We are delivering on one of the biggest challenges facing us in the coming years, tackling the historic neglect which has led to poor staying-on rates.
"The OECD provides welcome confirmation that we are beginning to turn the tide, with the UK now having over 1.5 million 16 to 18-year-olds in education and training, the highest number ever."
Despite a growing pool of graduates in the UK, the earnings value of a degree had been increasing, the OECD found.
The difference in salary between graduates and non-graduates in the UK was 58% in 2004 - up from 53% in 1997.
This suggests the market value of a degree is increasing.
Mr Schleicher said: "There does not seem to be a saturation in the market."
Only in the Czech Republic, Hungary, Poland, Switzerland and the US was a higher earnings premium placed on a degree.
According to the OECD, the UK "stands out with consistent rises in investment in education", from 4.3% of GDP in 1990 to 6.1% in 2003, most of which went into schools.
However, primary school classes averaged 24.3 pupils - the fourth largest.
The OECD said the money had been stretched, by children spending longer in lessons than in most countries and teachers being comparatively well paid.
But the picture for secondary schools was different, averaging only 21 pupils a class, compared with 24.1 for the other countries.
The average five-year-old in the UK could expect to spend 21.7 years in education, the highest amount of all countries, along with Australia.
The UK's shadow education secretary, David Willetts, said the report was evidence of how Britain was becoming a "divided society".
He said: "University graduate employment rates are rising where as employment rates for those without upper secondary education are falling.
"The Government¿s limited progress at getting pupils to stay on in upper secondary education is a major barrier to increasing access to university. These figures help to explain why social mobility is falling under Labour."