The number of young people available to go into higher education in England will decline sharply in the next decade, population estimates suggest.
The impact is less when social background is factored in
But the fall will be mainly in the social groups least likely to go to university, official figures show.
So the impact on universities would be less than might be thought, the Higher Education Policy Institute said.
The government rejects the think tank's analysis as "pessimistic" and says its policies will widen participation.
Last year the same group produced a report that sought to track the trend to 2015.
This predicted a rise in student numbers to 2010 then a sharp decline, so that by 2015 there would be only slightly more students than this year.
Having now extended this to 2020, Hepi says it has confirmed there is no chance of the government meeting its aim of having 50% participation by 2010.
It says GCSE results have improved, but in the past this has not led to more youngsters staying in education, doing A-levels a few years later, then going to university.
The effect of social class is important, though.
If the decline in numbers were equally spread, the number of young entrants in 2019 would be 11% down on what it is now.
When the social differential is taken into account, the reduction is just 2%.
Hepi acknowledges that the projections might prove to be wrong.
For example, the impact of higher tuition fees is unknown.
And the government is taking steps to try to get young people from a wider social range to go into higher education.
But on the face of it, participation among working class youngsters looks set to decline, tracking their birth rate.
The Higher Education Minister Bill Rammell said: "It is incredibly pessimistic and unjustified to suggest that schools will barely improve attainment or increase higher education participation over the next 15 years.
"We are also confident that the proportion of students from poorer backgrounds will increase."
Government changes - which bring in higher fees - would encourage people to go to university, he said.
Graduates would not have to repay their debts until they were earning £15,000. The poorest students would get financial help.
'Social and economic necessity'
"The Hepi research fails to consider the likely growth in non-traditional students like part-time students, or in postgraduates - English entrants in these two groups have increased by some 16,000 since 1999."
Mr Rammell repeated that the 50% target was "a social and economic necessity" and changes to 14-19 education were designed to raise the staying-on rate.
Earlier this month the Higher Education Funding Council for England said the current growth in student numbers meant the goal would not be met by 2010.
Shadow Minister for Higher Education, Boris Johnson said: "The aim of getting more young people to university is right - but we will never achieve 50% or more when only 44% of 15 year olds are achieving 5 or more GCSE's at grades A*-C, including English and Maths.
"It is madness that so many students are arriving at University in need of basic tuition of English and Maths."