Graduates from state schools earn less than their private school counterparts - even if they leave university with the same degree in the same subject and go into the same occupation.

A study by the Institute for Fiscal Studies shows a six per cent gap in earnings between state and private school alumni - equivalent to £1,500 a year - even when their qualifications are exactly the same.

The findings indicate that the benefits of a privileged upbringing and expensive schooling extend well beyond academic achievement, and help secure advancement throughout adult life.

Overall, private school graduates earn 17 per cent (£4,500) more than those from state schools, but part of this could be attributed to them being more likely to be from the country’s more selective universities and opting for higher paid jobs.

However the fact that a pay gap exists even between graduates in the same occupations led the researchers to ask whether the old boy network is still a key influence when it comes to employers deciding which candidates should be offered jobs.

“Education is often regarded as a route to social mobility,” said lead researcher Dr Claire Crawford, assistant professor of economics at the University of Warwick and a research fellow at the IFS.

“But our research shows that, even amongst those who succeed in obtaining a degree, family background - and in particular the type of school they went to - continues to influence their success in the work place.”

Researchers looked at the earnings of graduates both six months and 3.5 years after students had graduated.

They showed a seven per cent gap in pay with those who graduated with the same degree from the same university, dropping to six per cent if they actually went into the same profession.

“There are many reasons why we might expect graduates from the private sector to earn more, including access to particular social networks or having better non-cognitive skills,” continues the research.

However it adds: “Whilst there is much speculation about whether private schools do inculcate better non-cognitive skills in children, there is to our knowledge no robust evidence on whether this is the case.