The number of people with postgraduate qualifications has almost trebled since the mid-1990s, as employers expect more from potential employees, research by the Sutton Trust suggests.
A degree may no longer be enough for graduates who want to stand out in the job market, according to new research.
But the study raises concerns that students from lower and middle-class families are being priced out, leaving postgraduate study the preserve of the better-off.
The research, by academics at the London School of Economics and Surrey University, looked at levels of education in Britain and the US over time, and the wage premium of holding higher qualifications.
The findings show that more than one in 10 (11%) of people aged 26 to 60 in Britain now hold a postgraduate qualification, compared to 4% in 1996. It means that there are now 2.1 million people in Britain holding more than an undergraduate degree, compared to 600,000 around 17 years ago.
"In the past, employers used to accept O-levels or A-levels for many jobs," the report said. "More recently, a Bachelor's degree was expected. Now, graduates seek to distinguish themselves increasingly by acquiring a postgraduate degree."
Those who stay in education after gaining their first degree can expect to earn significantly more, the report said. A graduate with a Master's degree can expect to earn £5,500 more a year - around £200,000 over a 40-year working life - than someone who only has an undergraduate degree, it concluded.
The Sutton Trust said it was concerned that the rise in numbers gaining postgraduate qualifications could fuel inequalities, making it harder for those from less advantaged backgrounds to gain entry to certain professions.
In his foreword to the report, Sutton Trust chairman Sir Peter Lampl said: "Of course, a better-educated workforce should be good for Britain. Brainpower is what adds value in today's economy.
"But it is essential that this should not come at the expense of widening inequalities of access to these professions."