The cost of postgraduate university courses could become an extra obstacle to social mobility, warns a study from the Sutton Trust charity.
It says if students need postgraduate degrees for a tough jobs market, poor students should not be priced out.
There are fears that increased undergraduate tuition fees could deter people from staying on at university.
Postgraduate courses could become the "preserve of the better off student", says trust chairman, Sir Peter Lampl.
The study, carried out by researchers at the London School of Economics and Surrey University, looked at the rise in postgraduate numbers - and how it might affect fair access to jobs.
The proportion of people of working age in Britain with a postgraduate qualification has climbed rapidly - up to 11% from 4% in 1996.
The study found that a postgraduate degree remained linked to higher earnings, worth on average more than an extra £5,000 per year compared with someone who only had an undergraduate degree.
But the report raises concerns that if employers increasingly want to recruit people with postgraduate degrees, that such courses should not be limited to wealthier students.
In particular, there is a worry that if students have had to pay up to £9,000 per year for three years they are less likely to want to take on the financial burden of even more years of university.
"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," says Sir Peter.
"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," says Sir Peter, describing this inflation of academic expectations.
"Of course, a better educated workforce should be good for Britain. But it is essential that this should not come at the expense of widening inequalities of access to these professions.
"Postgraduate study is becoming increasingly the preserve of the better off student, both from home and abroad."
The Sutton Trust says there should be a better system for providing financial assistance to students wanting to stay on for postgraduate courses.