Some 70% of the current student population are aged 18 to 20
Some universities will face closure or merger as they struggle to compete for a dwindling number of students over the next 20 years, vice-chancellors warn.
A report for umbrella body Universities UK says unless institutions adapt quickly to the changing demographics, some institutions will become unviable.
The number of 18 to 20-year-olds is set to fall sharply between 2009 and 2027.
This means universities could face a smaller demand for places and hence a drop in public funding, it says.
The Universities UK report looks at three different scenarios predicting what will happen if institutions react in different ways to the changing demographics and a more difficult economic climate.
Predicted fall in student numbers between 2006 and 2027
England - minus 4.8%
Wales - minus 8.5%
Scotland - minus 10.9%
Northern Ireland - minus 13.2%
UK - minus 5.9%
In the first scenario, where universities are slow to adapt, student demand for places changes in line with population changes and funding levels fall.
Universities have to compete with each other for students leading to some reducing entry requirements and a loss of reputation for the UK system.
Although there will be an increased number of international students and more part-time and post-graduate students, ultimately some institutions will become unviable, it says.
In the second scenario, non-traditional private providers enter the market pace and "cherry pick" course areas with low entry costs.
A greater increase in e-learning also leads to partnerships with private firms.
But the increased competition leads to a "major reconfiguration" of the university sector with fewer large institutions and larger number of small specialist institutions.
In this scenario, damage to the education system is predicted as private providers gain degree awarding powers and a small number of elite institutions seek to leave the publicly funded sector.
In the third scenario, the university sector becomes more employer-driven and flexible and there is full development of technology-based learning thanks to public and private investment.
Most students end up studying part-time on a virtual basis while they continue to work, but full-time undergraduate study does remain part of the system.
This leads to universities grouping together strategically with employers and establishing themselves as major regional providers along side further education colleges.
Again, private providers cherry-pick vocational provision which will net them substantial profits and they also take over failing institutions.
But it leads to "extreme stratification" of the system and reduced opportunities for many young people.
Professor Rick Trainor, president of Universities UK, said the report was commissioned to help institutions consider the potential impact of longer-term trends and changes in demand.
"It gives institutions advance notice of the challenges that lie ahead so that they are well placed to anticipate these changes.
"Universities will be best placed to meet these challenges if the sector remains lightly regulated and is free to respond flexibly to changing student markets."
Baroness Delyth Morgan, Minister for Students, said: "Never has higher education been more important to the nationís success, and in the future, it will be even more so. It is vital to maintain the UKís standing over the next 10 - 15 years, that we create a framework for the expansion and development of our world class higher education sector.
"We welcome this thought provoking report which will form an important part of the debate on the future of higher education. While the scenarios donít necessarily represent what will actually happen, they do help to identify the sorts of challenges the higher education sector might face over the next 20 years."