A degree may no longer be enough for graduates who want to stand out in the job market, according to new research.
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.
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 per cent) of people aged 26 to 60 in Britain now hold a postgraduate qualification, compared to 4 per cent 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.
"But as the requirements of the labour market have become more demanding, this has exacerbated educational inequalities as workers with postgraduate degrees increasingly come from richer family backgrounds."
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: "When I was growing up, there were many professions that were open to young people with good A-levels.
"More recently, an undergraduate degree has become essential for many of those careers. Now we find that a postgraduate degree is increasingly expected."
He added: "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."
Sir Peter warned: "Postgraduate study is becoming increasingly the preserve of the better off student, both from home and abroad.
"That situation will be further exacerbated with £9,000 annual undergraduate fees for English students.
"Graduates facing debts in excess of £40,000 through undergraduate student loans are likely to see the prospect of funding a further £20,000 a year in fees and living costs, without having access to student loans, truly daunting."
The Sutton Trust believes that £9,000 fees are too high, and should be means tested, Sir Peter said.
It is calling for the government, professional bodies and universities to develop measures such as bursaries and a targeted state-backed loan scheme, to help students from lower-income families to continue their studies without racking up large debts.
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