Three-and-a-half years after leaving university, those who went to a fee-paying school take home almost £4,500 more, according to a research brief published by the Sutton Trust and upReach.
Trust chairman Sir Peter Lampl said the findings show that graduates from less privileged backgrounds face a disadvantage when it comes to pay increases.
The brief, based on research by the National Institute for Economic and Social Research, says that six months after graduation, those working in a high-status job who attended private school were earning £24,066 a year on average, while those who were state educated were taking home £22,735 - a difference of around £1,300.
After around three years in the workplace, the gap widens, with former private school students earning around 14% more - £36,036 on average, compared to £31,586 for someone who went to state school, a gap of £4,450.
The study suggests that half of the earnings difference is down to factors such as a graduate's prior achievement and the type of university they attended.
But it adds that there are other issues to take into account, suggesting: "A plausible explanation is that non-academic skills such as articulacy or assertiveness could play an important role in accessing high-status jobs and career progression once in employment."
Previous research published by the trust and upReach, an organisation supporting less privileged students to secure top jobs, concluded that those from more disadvantaged backgrounds are less likely to enter elite professions such as law or finance.
Sir Peter said: "We know that graduates from less privileged backgrounds are under-represented in the top professions but today's research shows that they face disadvantage when it comes to pay progression too.
"This new research shows us how vital it is that firms do more to improve social mobility through their recruitment practices. Enabling greater access to a wider pool of diverse talent will deliver real benefits for employers and employees alike."
More on AOL Money:
University tuition fees could be £10,000 a year by 2020
What are the prospects for this year's graduates?
Student squanders £58,000 college fund: blames parents